Hey, from one ace-spec to another! So... guilt? Is not the msot common ace experience. I'll develop later. Feeling bad, maybe. Depends on what you mean by "bad", really.
From my experience and people who I've talked to, it would be more like "THIS feels bad when it's with me" (as in, it is physically painful, no arousal, or maybe just overall super uncomfortable - you can't get un the mood when it's actually you with another person, maybe it even turns you off, maybe it even makes you feel repulsed).
Which by the way does not mean at all that every ace person feels negative towards sex. Some people can be ace or ace-spec and really enjoy having sex. Mostly, ace people are the ones who don't feel sexual attraction towards other people.
Some ace people feel arousal, they can enjoy themselves alone, or even with others, because maybe the physical sensation of sex is actually pleasant! This does not stop them from being ace, because they still don't feel actually attracted to other people. So the "hey look how HOT that person is" feels alien (in a sexual sense, you can still admire beauty and acknowledge someone is hot, but you don't get a sexual thing from looking at them).
Some ace people don't ever feel arousal, but are not repulsed or against having sex. Some are neutral and see it more like going shopping with your friends or whatnot, like. Sure I'll do it if you want, it doesn't bother me and I want you to feel good.
Some ace people are legit sex repulsed, they feel very awkward and unpleasant around the idea of actual sex, they don't want to ever do that, they hate sex scenes in movies, sometimes even the mention of it makes them repulsed.
Plus many other combos of traits described above, and the complicator that some may enjoy or be aroused by sex only to a certain degree of exposure (e.g., if it's a cartoon or in written form, that's fine, but if it's live action, nope). There's several layers of nuance, each ace person is different. The common denominator? No sexual attraction.
But going back to guilt. Guilt is a complex feeling. Getting guilty due to sex CAN happen to ace people, of course, but it's not the common defining experience. So don't read this as in "hey you're not really ace >:(", please! Just as a "hey, you should go after the origin of this guilt to better understand yourself and maybe even get to enjoy sex, if it's your thing".
If it's guilt for enjoying it when you "shouldn't because your ace", you can throw that out of the window. Ace people can enjoy sex. It doesn't make them any less ace.
If it's guilt because of any religious context, due to negative past experiences, due to social tabu, etc... then hey, this has nothing to do with your ace-ness. Society has a weird ass relationship with sex and loves to paint it as immoral and "naughty" and etc etc. It's something worth trying to deconstruct, but not easy.
Also!!!! In case this guilt comes after the act, not during it, you may wanna look into Postcoital Dysphoria. It's a phenomenon that affects people of all genders and sexualities.
All that to say that being ace is usually not about not being able to "allow yourself" to enjoy it. If your issue is that you blame yourself and won't allow yourself to feel good about sex, then you should definitely try to go after the reason for it and free yourself from that guilt.
Being ace shouldn't lead you to feel guilty about enjoying things, so don't avoid going after the source of that guilt just because you may be ace. Asexuality shouldn't feel bad.
Anyways!! Hope this helps in any way on your questioning journey. Remember: having sex doesn't make you less ace.