I asked him about this and it finally came out that he is living with his ex-girlfriend.Basically, she is living there rent free and has been for 10 months, so she can save enough to move out (we live in NYC).
What I've noticed, though, is that every person I've heard espouse this worldview was straight.
Yes, you may (as many people tend to) get completely wrapped up in your own feelings and give the middle finger to anyone who tries to tell you otherwise, but if one of your besties decided to start humping your ex, would you be supportive or forgiving?
The sheer concept of seeing one of my mates with an ex is something that’d affect me for a number of reasons.
Queer communities are often small and insular, and once you've found one, you tend to hold on to it for dear life.
It's difficult to meet people you're romantically interested in beyond an already-defined circle, and outside of your city's queer scene, most people you run into are likely to be straight.