I'm not sure that that is necessarily true. Certainly I think that we can all pretty much agree that Buffy wouldn't be the sort of person to live any sort of celibate life for very long. We've seen that illustrated perfectly with Satsu in Season 8. My point is, Buffy would have had a relationship with someone in high school, whether Angel was there or not. Buffy's relationship with Spike began with them being enemies and progressing through slow stages from enemy to informant, to barely trusted ally, to trusted ally, to confidante and trusted ally, to lover, to trusted friend, ally, and someone who she could love. I seriously doubt that not having a relationship with Angel would have changed any of that significantly, unless it actually made a relationship with Spike somewhat easier, since the vampire with a soul/vampire without a soul issue would never have been a factor.