I think that the main reason that the Buffy/Angel sex scene was so toned down was that Buffy was a minor having sex with an obviously much older man, not as in 200 year old vampire old as much as 25 year old man with 16 year old girl. In other words, she was jail bait, and Angel was committing statutory rape. I don't think thayt it was an accident that, after that onw time, Buffy remained celibate until entering college, when there could be no doubt that she was of legal age. I'm actually surprised that the infamous Censors allowed Joss to have that one scene. They would have gone into apoplexy if he had made it a regular thing.
And it just occurred to me that Buffy's first sex scene, in "Innocence," and what a lot of us consider her last sex scene, in "Chosen," (unless you think that Buffy and Spike spent that last night playing checkers) were both left entirely up to the viewers' imaginations, as both scenes faded to black before things got very steamy.