It was a good pushback... probably.
You suggest she accept him anally, she basically said "you first". IOW, you asked, she told you the price.
Sounds like it could be a place to start negotiation. How much do you want to see her do it? Enough to try it yourself?
(I'm pretty sure the answer is...
Incidental contact while trying to please the same woman is not going to make you gay. I wouldn't worry about it, just get in there and it'll all be good.
Good for you! My girlfriend in college liked it when we had company (male or female) and everyone got involved in basically a free for all.
Except my roommate. For some reason he simply could not abide semen in his mouth. Sucking dick was fine, being fucked was fine, fucking any of the five...
Now let's be fair here, this was your first time. She's presumably had way more practice, on more dicks.
Get out there and get some practice, get trained up, and then compare!