Yes, it was Ramsay’s idea. It was very clear.

Myranda: You said, you’ll marry me.

Ramsay: And I meant it!

I can’t imagine Myranda forcing anything on Ramsay. It was his decision and the fact he tolerated her complaining and his only reaction was rolling eyes and attempts of cheer her up (“don’t worry. I’ll have a plenty time for you”) were proofs that he was really enchanted with her. He always looked at her with some fascination and admiration (on the hunt, during sex scene while Yara’s attack or after the moment Myranda bit his lower lip).

Ramsay through years tried to impress his father with no succes. Probably finally he was frustrated and resigned. No matter what he did, there was no chance to became a real Bolton. Year after year passed and Ramsay started to think about the second way for his life: with the only woman he cared and who understood him. I think he proposed her when he was in an awful mood. Roose had humiliate him again and angry Ramsay looked for cheer up in Myranda’s arms. Maybe he said impulsive something about marriage maybe he thought about that from long time. The fact is: he really wanted it and yeah… If Ramsay felt something to someone it was for sure to Myranda.

Please notice the way he is looking at her:

so YES, I think she was the woman of his life and he has proposed her for real.