I'm not upset. Just curious as to why your view flip-flops based on gender. You wouldn't go down on a friend of yours, but you'd have sex with her regardless of the fact that it would change the relationship. What if that change complicated the friendship (as you claim to say it would)? You say you would still do it. At least that is what I gathered.
It's only inevitable if you make the decision. Banging my friends in any way is not an option because I want to keep them in my life, as friends. Sex is not worth losing my close friends, or complicating those friendships. There is no such thing as an "accident". Similar to how a drunk person may cheat on their SO; it's still a conscious decision to move forward.