Why has my 20 year old son has not shown any signs of liking girls?

My oldest son who is soon to be 20 has not shown any signs of liking girls. My son is a very very good son in fact his father and I kid around saying that he is not normal because he is such a nice person in every way we see it. He is very respectful, polite, courteous, generous and considerate and also very smart. He is now attending his second year of college and is doing very well with everything. Except i have never seen him hanging out with any girls, talking on the phone with any girls and bringing home any girls or even talking about girls at all, like most guys do. Most guys talk about seeing a girl at a store and how good looking she was or this and that. My son has a group of friends who he met in high school and still hangs out with them because they all go to college together. I have met all of them and I know that none of them appear to be gay. He talks on the phone with some of them about classes they have in school and or video games and movies, etc. My son says some of the guys have girlfriends and some don't. My other son who is 17 is hanging out with a couple of girls and i hear him talking on his cell phone with them and has even brought a few female friends to the house. I have asked my 20 year old son what kind of girls he likes in a way because I am trying to find out something that can tell me about his thoughts or feelings and he just tells me he likes any type of girl as long as they pretty he says beauty is in the eye of the beholder, which is true, but still i end up knowing nothing. When people ask me how my son is they always also ask me if he has a girlfriend which I always used to say "no" he just has a group of friends, girls and guys. They always seem to be surprised and make me feel uneasy. Now when they ask i lie and say he does just so they leave me alone. My son seems to think that I am homophobic because i always say things that make him think i don't like gays or lesbians and one day he asked me jokingly what would I think if he told me he was gay and I reacted in a very emotional and hysterical and during this whole conversation he was laughing and I asked him "what are you trying to tell me?" he said "Nothing I just know how you get feel about that and I like to see you scare. He says i overreact to many things. Things like these make me feel uneasy about thinking of a possibility of him not liking girls, and being gay and being afraid of me and his dad. Or maybe he is just very shy and does not know how to open up to girls and or talk to girls. He is not the person to initiate a conversation with people. He and I are somewhat alike. We always say to each other that if someone does not talk to us, we won't go the extra step to talk to people. I don't know what to think. We tend to avoid crowds and loud occasions. When we do have to go to parties is because we have to as an obligation and if we can kick it home we rather stay home. I hoping that this is the reason why. I welcome your opinion.