Think a lot of women are more accepting of it nowadays: it's a lot more mainstream than it used to be. Only problem is if she has it in her head that it's going to hurt, it probably will since she's all tensed up, so remind her that you're going to take it slow, with lots of warm up, and that at the slightest sign from her you'll stop. Maybe even suggest that she take control cowgirl-style, since then she's in complete control of how fast or deep to go. As long as you make it clear that the options are all hers, and she still has control (and especially that she can back out at any time without feeling like she's let you down!), then she'll be less worried about it all going wrong. Normally it's that terror of 'the point of no return' that gets you, so just emphasise that you can stoop any time, literally, even if you've gotten in and been thrusting away happily for five minutes already, if she decides it's coming out, out it'll come. Probably all stuff you'd do anyway, but just as long as she hears it.
Or you could tell her that exercising her sphincter with daily stretches should make her less likely to kak herself when she's old and decrepit. Nothing relaxes you like a good laugh and punching your partner. :)