No I don't think all gay couples look a like but a lot of them do. here is my theory and yes, I look VERY similar to my boyfriend, as a matter of fact, we look more similar now then when we met over a year ago. We are frequently mistaken for brothers. He has blue eyes, I have brown and he is 6" taller than me.
Theory #1: Most people wake up, shower, fix their hair, wear clothes, that they feel make them look attractive. Therefor if I think I look attractive as a buzzed head bearded guy, I likely find that look attractive on other guys or I want to look more like the guys I find attractive. Meaning that it is easy to be attracted to a look whether its on someone else or yourself!
Theory #2: As gay couple grow more close and live together, it is far more likely that they end up wearing each others clothes, then shopping at stores together and buy clothes that again look similar.
Also, one thing a lot of people don't realize is that gays and lesbians really segregate themselves into groups within their own community. There are the Bears who are mostly hairy stocky guys, mostly bearded. There are twinks, who are younger thin guys, there are the leather dudes etc... so that becomes your social circle therefore, thats where you meet guys to date and become a couple with. There are even internet websites targetted to for specific groups. bear411.com is one to meet bears and pretty much everyone on that site could be described the same way. Recon.com is to meet guys into leather - same deal, everyone sort of looks the same!
Hope that helps. My advice as you come into your sexuality is, fall in love and be attracted to whoever you want to be attracted to whether or not they look liked you or are the same gender or different.