With zero knowledge of circumstance (and there are almost always circumstances) though, to say that just because you're older you're "manipulating", is idiotic. I'm not saying that doesn't happen, it does, but I never had anything but the best of intentions when I dated that girl, and when it ended, it wasn't because I wanted it too, and if anything I was the one more broken up by it. I dated her because I really liked her, and treated the same as if I were dating someone my own age. There was no "power imbalance" because I respected her and valued her, just as she did to me. Angel always respected Buffy. Sometimes he worried about her and did what he felt was best, just as Buffy did with him, and I'm not saying he was always right, but saying he had no respect for her is more than a stretch.
You're right, in California the age of consent is legally 18. However, this clearly is not a clear-cut morality law where you're either right or you're wrong. Everywhere in Canada, the age of consent is 16 or 17. So if Angel sleeps with Buffy in California he's a rapist, but if he does it in Toronto everything's perfectly fine? You can't realistically say there isn't some ambiguity there..