Confessing to a friend

So I have a really good friend who I didn't know very well until recently. This friend is in a relationship with a girl, and they seem really happy together. But every time my friend brings up the girlfriend, I get really anxious and fidgety. I don't know what to do because I haven't felt like this before and I really don't want to get in between the two, but it's putting a strain on our personal friendship and I really need to let go of it all.

And to top it all off, there are some really homophobic people in my year, so I don't want to draw any attention to the matter in case it backfires on my friend.

Any advice on what to do in this situation would be greatly appreciated.