She is not a lesbian, she is bisexual.

You can not fuck a lesbian (legally) under the typical "only likes girls" definition.

There are a few possibilities here.

You're afraid of not being good enough because she usually only gets with women, women know what women like because women know precisely how it feels.

You feel like the outcome of what you're going to do will impact whether she likes men in future.

You feel like you're doing something wrong, that you have manipulated her against her primary sexuality urges.

If the top two, it really doesn't matter. If the latter, any manipulation is an issue but there's no such thing as motivating her against primary sexuality urges, there is no base right or wrong with inter-gender sexuality, just what people feel sexually attracted to and not.