I don't know if I can help at all, or if this advice might work or not, but some things have occurred to me while reading this thread and I offer them in the hopes it helps you improve your relationship with your mom (and hopefully continue watching Buffy and W/T with your mom's acceptance).Your mom sounds a little closed-minded, and she might never accept things her beliefs tell her are "wrong". But perhaps you can open her mind a little. You might start by trying to be more mature and show your mom that you can learn from the good examples on Buffy without mimicking the bad ones. If your mom can see that you can distinguish good behavior from bad behavior on the show, she might be less concerned that it will be a bad influence on you.
Willow and Tara have had some fights recently (and one towards the end of season 5 which was their first ever), but they don't throw things when they're angry and they don't scream at each other even when they are upset and somewhat frustrated. (Willow used magick to throw things at Glory last season, but that's part of the fantasy evil-fighting side of the show and not meant as a literal example of how to resolve conflicts in real life). Perhaps you could learn from their good example and try talking about things with your mom in a mature fashion. You both probably have things to learn and you can probably help each other if you're both willing to make the effort to be reasonable.
Try talking with your mom about the show and about Willow and Tara's relationship. Don't just start listing points but set up that you want to discuss each other's feelings about the show. You want to understand her feelings, and you want her to understand yours. You need to talk with each other, not just at each other, or you'll probably both just continue being frustrated and angry with each other. Does that make sense?
For example, think about episodes that have taught you something. In season 4 Buffy slept with a guy she'd just recently met and then the next day he pretty much blew her off and had no real feelings for her, and she was quite hurt. Your mom would probably be appalled at pre-marital sex and especially when people barely know each other, but point out that Buffy's mistake teaches you something (try to be specific), so maybe you're less likely to make that mistake yourself. (Of course, it's best to use examples you've actually seen and especially lessons you really learned (though even an example of a lesson you already knew works as an example of what the show can teach you), so if you didn't see that episode find other examples.) If your mom can understand that BtVS does teach important life lessons, she might be more accepting of the show.
Also ask her what she objects to. Try to understand her point of view, and see if there's something you could point out that might soften her less-reasonable views. Maybe try to point out the flaws in her thinking by gently asking questions ("But if two people love each other, why is it wrong just because they're both women?"). If she insists on declarations like "It just is" then point out that that doesn't help you understand her views. This general approach can backfire ("Don't get smart!"--so watch your tone of voice and try to be gentle and curious; you're trying to get her to explain her feelings, and thus to think about them herself rather than just cling to an unsupported belief because she's comfortable with it). But yelling at each other is probably worse, so it's worth a try.
If the discussions go well, invite her to watch and discuss the show with you. Try to find examples of choices the characters make and why you think they were good or why they were bad (most importantly about their regular lives, but metaphorical lessons can be drawn from most of the supernatural stuff too at least if you think about it). If your mom can discuss the show with you, she can share her values and help you avoid being "improperly influenced" by it, and she can see that you take good lessons from it and that you know (perhaps with her guidance) what examples are not meant to be emulated. (They don't intend people to practice wicca or try to do magick just because it's on the show, any more than they intend people to go out and slay vampires and monsters or solve conflicts through violence. And a lot of Buffy and Spike's recent behavior is meant as an example of an unhealthy relationship, not one to go emulate.)
Of course some of her "guidance" might be less helpful, and you won't necessarily agree with her on many things. It's important that you agree to be mature and reasonable with each other and both understand that you might not agree with each other on some things. Instead of arguing over them, ask gently-probing questions to better understand (and indirectly challenge each other to back up your views with reasons and rational thinking).
But remember, it's a discussion not a debate or a contest to see who can win an argument, the point is to help both of you by better understanding each other and improving your relationship with each other. If you can also learn from each other, that's a bonus.
