Immaturity has a lot to do with "being" gay. There are many people who simply never grow up because of bad experiences with a parent of the opposite sex - which they project onto anyone of the opposite sex. I have known women with abusive fathers who "became" lesbians beacuse they thought they were "avenging" their mothers.
And then there are other people, both male and female, who never seem to get out of the Peter Pan phase. Forever adolescent.
In any case, gay couples do look a lot alike. Of course, that's true with some older straight couples, too; they say people even pick out their dogs on the basis of some resemblance. However, two people who resemble each other and are of the same sex are going to look like near identical twins... Kinda creepy.