A good rule is to only do what
you're comfortable with, and this goes both ways. Otherwise, it'll lead to bad things with one thinking the other is taking advantage of them.
And while you may feel inclined to do it to make her happy with the hopes that you can then get her to do something you want, if you do intend to have her repay you in kind, you'll wanna hash out that agreement beforehand. Otherwise,
You Might not get a return gesture from her. But her willingness to repay you in kind will say a lot about her and her true motivations.
All that being said, it's just a toy. I've licked and sucked on the toys (some more realistic than others) of girls I've been with, and if they've just used it on themselves (which is the best time to do it), it tastes like them. Not only can it give her an erotic image from your actions, but it also shows how much you like her taste if you're wanting to orally clean her toys. I've also been with a girl who as a part of her trying anal, wanted to share in the experience of trying it, and talked me into her trying things back there on me, as it was her trying to give me more pleasure because she loved me. Now while I was only ever comfortable enough with her using parts of her body and not toys in the time we were together, because of my experience with her, I was comfortable enough with the thought of things happening back there that when the girl I'd eventually marry inquired about also using toys on me (having already pleasurably used her body many times), I was okay with it. But I will give her credit for calling the strap-ons she'd use on me and have me suck (one for each job) 'her big clits' and using toys that were basic phallic objects, and not fully anatomically correct. She didn't want me to be uncomfortable, and she actually liked the idea that I was sucking her 'big clit' or that she was using her 'big clit' on me better than the initial idea of seeing me sucking or getting ass-fucked by a cock. She also made sure that what she did was pleasurable to me so that I'd be more willing to do it again, and since it also made her happy, she was wanting to do things to make me happy to repay me.
The point of that story being that you could compromise with a less realistic toy that she has just used on herself to give her flavor, if it would make you more comfortable, and your girl should be willing to make you comfortable in her fantasy.