I've told my GF that I want to suck cock. She says it's okay with her if I go try. How do I make it not feel like cheating?

We've been together for 3 years now, and she's extremely supportive and loving. This is a fetish/fantasy that I've had for decades... She says she supports me, and I believe her. But I can't actually get the nerves up to try because it feels like cheating..

Any advice?