Because sex feels really good and having sex with someone you really like, for the first time (with that person), is really exciting and fluttery... it just depends on the people! Of course sometimes slow burn sexual tension buildup is nice, but a lot of the time in manga, and sometimes irl, the building sexual tension is while it's leading up to getting together. So when they get together, that's the point where the sexual tension gets released! To be honest, I'm like that in real life too so it actually seems realistic to me. But like I said, it just depends on the person I guess ヾ(❀╹◡╹)ノ~
Wow just realised I'm writing this over a year after the original post!
i've been thinking this for a while but why does everybody jump straight into sex? just personally, but if i actually like someone i'd want to build up a bit before that. (if it's someone i don't give a shit about though, it's whatever)