We can first look at his interaction with Tav after Haarlep. He is the only one who shows compassion to Tav after the deal they made saying :

"I know what it's like to lose control over your own body. It's a wretched thing."

This is one of the many windows into his past and experiences.

Astarion tried to enjoy sex in the beginning, but failed:

I thought the same,once. It didn't last.

This is said after Tav says they will try to enjoy their time while Haarlep uses their body.

From very early on, he didn't want to sleep with any of his conquests. I'd go as far as to say he never did.

I truly wanted you - is the ony lie in this sentence.

This is from his conversation with Sebastian who he slept with around 170 years ago .

He also says enjoyment didn't come into it.

Enjoyment? That didn't come into it. I never had a choice.

Now, that lack of enjoyment came from many factors. Here are some that he himself has mentioned :

1) Guilt

He felt guilt when sleeping with someone because he knew they'd die. As much as we see him talk about enjoying killing and murder in act 1, there are traces there that show he feels deeply wrong about his victims.

I found a darling boy who I couldn't bear to bring back to him. So I ran, instead of hurting that sweet man.

Your seething passions lead only to pleasure. - As opposed to pain, death, and misery I assume.

2) Lack of connection

Astarion was deeply lonely from not being able to connect with anyone.

Tell her about the years of loneliness and fear you felt living under Cazador.

3) Emotional numbing

Astarion talks of his encounters as pleasurable but emotionally numbing. He is also pretty clearly dissociating during sex.

- But I always felt nothing.

I know that dance better than anyone. But I always felt nothing.

I already lived an eternity devoted to pleasure, and it was rather dull.

- Astarion to Haarlep.

Probably his emotional numbing made it harder for him to reach the climax of things.

Most of them didn't even grant me temporary bliss.

4) Some even turned violent.

When asked "Did Sebastian hurt you in any way?" he responds with "He didn't do anything at all. I can't say the same of all of my victims."

Afterwards you felt empty? Alone? Ashamed?

Even though he is talking to Tav here, I see it as him directly referencing how he felt after a conquest. Immediately after he gives his advice that it’s best not to dwell on it. When asked if that works, he continues with : 

Well..."works" might be putting it a little strong but...makes it feel less terrible,at least.

- makes it feel less terrible,at least.

Which to me suggests he felt terrible after most of his encounters as well. 

And this part gets interesting. It is clear in the beginning of his relationship with Tav that he still struggles with wanting and enjoying sex. But he talks about how it felt different for him from the start.

That time was special to me.

For him, the enjoyment was not there with his victims mostly because of the lack of choice. 

With Tav it's different, probably because while still not romantic in the beginning, it was the first time he had a choice.

When asked if the nights together meant anything to him:

Of course they did- that's the problem.

When Tav coerces him into sex, it's the first time they take away his choice in the bedroom.

Which imo, is why he feels the way that he does after.

Despite his feelings and his choice, he still struggles with emotional numbing during sex, even with Tav.

He lashes out and diminishes what he was feeling because of the betrayal if the confession in Act 2 goes poorly or if it never happens at all.

In act 3, on his grave he feels safe enough and loved enough to try experiencing sex as a form of connection and expression of love. 

I feel safe with you. Seen. And whatever the future holds for me, I don't want to lose that.

There is a strong implication that this hasn't happened to him before with anyone, as he claims no one ever showed him kindness except Tav.

Astarion doesn't use the word 'sex' if he's genuine.

With Tav, after getting in a relationship, he uses many different ways to talk around it:

"Being close to someone." ;

"Haven't done...that in a while"; "Do this again" - in the brothel and with Halsin

"Cuddle" - Durge and in act 3 pre grave scene

"A night of passion" - on his grave

"How much we enjoy each other" - to Jaheira , etc

I think this is a tactic to distance sex from what they have because sex is the thing he was forced to do.  He only uses "sex" if the confession goes badly:

"There is no point to me if I won't have sex. My only talent. I'm fully aware."

He is also, in general, putting a lot of distance between Tav and his conquests before.  He can’t stand to think what would have happened if they’d met before, and his confession in the first place is because he can’t stand to keep treating Tav like a conquest. 

I find this also very funny because when he talks so brazenly about sex in act 1, and the contrast with how he talks about intimacy in Act 3 is starkly different.

Astarion and his own view of his past:

On more than one occasion, he refers to himself as a prostitute, or he references that he offered "services."

You have a type, don't you? Elven prostitutes? Again? It's rather emabarassing,dear.

I like this line because it's also one of the few moments he seems quite jealous.

Here are some more, no pics, sadly, as I couldn't find clips:

"What a world. I never dreamed I'd be on the paying end in a brothel." - said if platonic Tav takes him to the drow.

Origin Astarion:

A: "I was once in your position, in a way."

Sorn Orlith: What was it like?

A: "I try not to think about it any longer." or A: : "I was forced into it, but it sounds as if you weren't."

Sorn Orlith: My... my apologies for prying.

"Now, though, I'm hardly going to give you a treat when you haven't helped me in exchange for my excellent services." - to Tav when the act 2 confession didn't happen.

And on that note in the same breakup conversation we get more context:

T: "Is that all you ever saw us as? A transaction?"

A: "But this is what I do. Those are the talents I have."

A: "I can guarantee, it stings far more to have to pay your way through the night with your own body for hundreds of years."

It further solidifies that in the beginning, his view on the relationship with Tav was transactional.

In a way, with this breakup, he is pushing the relationship in Act 3 to the way it was in Act 1 as there was no emotional intimacy achieved. At least a mutual one. I believe at this point he had developed feelings, but wasn't feeling the reciprocity, as to get here that means Tav didn't help him in his personal quest. (or meet Araj)

Last interesting detail, Origin Astarion tells this to Nym: "Just don't tell me what to do... It brings back bad memories."

The context is in the bedroom.

Overall this is all I could find on his sexual/romantic history, and my thoughts on it. If there is anything I've missed please do link it, I'll add it in.

Some of these images are from youtube as I haven't done a lot of these choices, if you'd like a link to the videos I am putting one below :

ascended Astarion catching Tav cheating

Astarion talk after Haarlep

Astarion and Nym

Astarion talk to Sebastian (origin)

The bad interaction in Act 2

Astarion talk to Naoise