When Buffy loses her virginity, Angel loses his soul for feeling happiness, and then turns on her. He becomes evil again and makes Buffy feel horrible about herself. Her situation with Spike is a little bit different, though. She openly enjoys her sex with Spike, which theoretically is forbidden. The repercussions of this are in Spike's attempted rape of Buffy. It is framed as a punishment for Buffy enjoying the sex. That said, afterwards in the following season, Spike seeks to get his soul back so that he will be worthy of Buffy's love. This is not the harsh consequence that usually follows a stereotypical Hollywood sexual interaction.
(I tried to find a clip on youtube of Buffy's fight with Angelus, but I couldn't find it--you can watch it on Netflix if you'd like to complement the description).
It is said that the producers added the rape scene specifically to make the viewers feel thats Spike is NOT worthy of her love, however if that's really the case, then why would he seek to become better for her afterwards? This show was also created to counter the stereotypical helpless female, but at times, like the post-sex incidents with Spike and Angel, it seems like that's exactly what Buffy is (in this way). Buffy is, of course, an excellent fighter, and at times the most powerful one out there. In my opinion, however, what happens off the battle field is just as important as what happens on it, in terms of character assessment.