First I don't want to catch something (aids, chlamydia, herpes, ...) so a few minutes of sex doesn't seem worth the risk. I know, at least according to popular media that puts me in the minority. If my attitude is common it's probably not easy to spot.
Second, I don't want to hurt people. I know they exist but 9 times out of 10 people I meet want a serious relationship and would be hurt if we slept together than then I called it quits. I wouldn't lie to them directly but they'd feel like I was lying to them if I say, slept with them 2-3 times and then stopped seeing them. So, I basically don't get into it in the first place unless I believe that I'd like to be with them longer term. Of those people that could be more casual usually issue 1 comes up. They sleep around so they are more likely to have something and plus I don't want my heart broken.
Third, I'm also someone who just wants one person not just for sex but for love and companionship so my attitude is bad when meeting / looking for people. My attitude is "can I see myself being with this person long term" instead of "would it be fun to be with this person for the moment". That answer is quite often "no, I can't see myself being with this person long term". I don't think that's that strange of a thought. A stereotype of a story is the person that doesn't think this way and has lots of failed relationships for choosing superficial qualities of attractiveness as their reason for getting into a relationship. But, at least they had a relationship.
Anyway, the sum of those 3 things and possibly some bad luck being in jobs which have few members of the opposite sex and friends that have few friends of the opposite sex means I haven't had a relationship for ~17 years. I hope every day that will change, I frequent 5 dating sites. I have matches. But getting something started seems really hard. I would do much better if I went to church (I'm atheist) or did some activity where I got to know people over time and become friends first. I've gone to several meetups but if they are truly about something I care about they usually have few members of the opposite sex. I've been to a few otherwise but it's been the same bad luck I guess. No one I'm interested in or no one that shows interest.