Okay, well, for starters, the same questions could be applied to heterosexuality. They key difference, of course, is that heterosexual relationships often result in reproduction, and therefore, the continuation of the human race. Two men or two women can't reproduce, so gay relationships aren't about sexual reproduction. This is the most frequently-cited nonreligious condemnation of homosexual behavior.
But in regards to whether or not it's "okay", without invoking religious dogma, seems kind of pointless. Implying that not reproducing is morally reprehensible would mean condemning not just homosexuals, but sterile or infertile men and women, or couples who simply aren't interested in having children. If the only negative side effect of a monogamous, loving, homosexual relationship is a lack of babies, then there really isn't a problem. I've liked dudes my whole life. Of course, as an adolescent, I too believed this was wrong and so tried to change it. All I accomplished was spending my teenage years in the closet while pretending to like girls and trying to jack of to pictures of naked women, only to have my horny mind want to look at dudes instead.