It goes from a sexy scene to having to be put in chastityWhen we write porn, we tend to exaggerate.
Guys get instant erections, and they remain rock hard for as long as required.
Women are immediately wet, never need lubrication.
Reality is of course very different.
In the end, how realistically you wanna portray a guy and his erection, how fast he gets one, whether he can "control" it, really just depends on the story you wanna tell.
Is it some dare / challenge thing? Like "how long can she tease him or how much skin can she show before he gets hard?"
Is he making out with a new girlfriend and trying not to get an erection because he's afraid she'll notice and be offended?
Is it a nude beach thing? New guy, naked around a bunch of attractive nude women and trying not to get an erection?
So many variables. In the end you can write it however you want really.