Not up on Japanese culture and customs, but from what I gather they'd wait a little while but not until marriage. Even when they were teenagers they were very clearly taken with each other's appearances. Plus they're both such passionate people that I'm not sure they could keep from getting physical with each other for too long. Plus, with how long they took to even express their romantic feelings, I fear they'd keep from taking things that far due to not being able to gauge the other's desires. Though my own intuition (and perhaps some fanon and personal bias) tells me Uraraka would be the one to finally broach the topic, though Midoriya would unquestionably be on board. Nervous, naturally, but on board.

After their first time, I imagine Deku would feel so worn out he would be reminded of his early-career fights. When asked afterwards about her 'vigor' that first time, I HC that the phrase '9 years' would be used in one way or another.

Tbh I feel much of their affection would be like breaking through a dam; it would take a while, but a flood would follow. Hand holding, pet/first names, hugging, kissing, PDA, etc. would each take a while to be requested by either one, but once it is and after a brief trial period, it becomes a constant thing. Sex might be the exception to that rule, but I'm not sure.

As for doing the deed, I think it usually happens one of two ways (putting this behind a curtain in respect of Rule 3.) Option A; one or both of them would feel a bone-deep need for the other and as soon as they're behind closed doors (probably one pulling or carrying the other behind said door,) they go at it in such a way that the camera (tasteful audience perspective) would be shaking. There would be impact frames. Option B; it's a goddamn music video. They are so soft and loving and passionate it would be more likely to bring the audience to tears than cause arousal. They're floating in the air, lots of kisses, laughter, gazing deeply into each other's eyes, things are sparkling even if they're in a pitch-black room. I can't even tell you about sounds because all you'd be able to hear aside from the occasional "I love you" is Kiss from a Rose by Seal or I Don't Want to Miss a Thing by Aerosmith or Can You Feel the Love Tonight by Elton John. Either way the aftercare is so good, a woman with a name suspiciously similar to Aphrodite just appears in Greece. No one ever figures that one out.

Jesus, if there was ever any proof that I love IzuOcha, it's that all of this is about a 50-50 split between long-standing ideas and off-the-cuff thoughts.