When a bisexual guy identifies as gay, it's typically because he's not romantically attracted to women. He can fuck women, but he doesn't fall in love with women. Most bisexual guys are the opposite of your (mostly) gay friend, i.e., they can fuck men but they don't fall in love with men, which is why most bi guys identify as (mostly) straight.
I'm wondering how Dan Savage knows that "most bisexual men... can fuck men but they don't fall in love with men." I'm further curious as to how he knows that "most bi guys identify as (mostly) straight." I'm not aware of any scientific studies that have come to these conclusions. My feeling here is that Savage is merely reasserting the tired old stereotypes he holds about bisexuals. Oh, great sage, tell us how you came to this knowledge. Was it an intense study of the M4M ads on Craigslist? The gossip among your friends? Were you burned in a previous relationship with a bisexual, and you believe it appropriate to take that bad experience and use it to generalize about all bisexuals? You don't know everything, Mr. Savage, and I wish you'd stop pretending you do.