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HEAVY Spoiler
It's true that the seme (ml) already knew that uke (mc) was lying about his experience with girls, but that's not the reason why he approached him. Since their first meeting, he has been quite attracted to uke (before uke even notices him). Even when they weren't close, when he learned that uke didn't like the smell of cigarettes, he quit smoking (the reason why uke never knew he smoked).
But for him (seme) he wasn't in love with uke, he just thought he was attracted to him, so when he approached him it was just to have a one night stand, but he quickly abandoned this plan, because he is too much attracted to MC.
At this point in the story, he already knew he wanted to be with MC, and quite quickly, he will be completely in love and he's the obsessive type. So don't worry now that they're together, he will never do something that could cause a misunderstanding, he is too much in love with the uke for that. He went crazy when he thought mc wanted to break up with him (uke learned of his past lies), he couldn't sleep for days out of fear, and even if mc cheated on him he would forgive him because he can't imagine spending even one day without mc. He said something like "I can only think about you. You made me like this. Please do something", the man is very obsessed, and he's very panicked when he wake up and mc isn't near him. They have a life and death kind of relationship.
I think it was love at first sight for ml (even if he didn't notice).