I am 17, and I think I have only ejaculated once. That was a good many of months ago. I have gotten pre-cum when I am aroused, but I do not think I have ejaculated since. I do not ever want to masturbate because I think it is sinful. I do not think nocturnal emissions are wrong because they are involuntary. However, it seems like I have never had one. I sleep on my back and my boxers are pretty tight is this hindering my body's cycle? If so, how do I have wet dreams so that I do not feel so "horny" all the time?
Men who take certain medications, especially anti-depressants and other psychological medications, may find themselves with a decreased libido (the desire for sex). It sounds like your libido is there. Since you ejaculated once, the system works. About the only thing I can guess is that your semen production is very slow or you are dripping at night instead of having full orgasm and ejaculations.
Your underwear would not impact your ability or need to ejaculate. There was one study decades ago that concluded that tight briefs might cause low sperm count, but that doesn't mean lower amounts of semen.
What I would suggest doing is talking to either your general doctor or a urologist about this. It may be just the way you are, or there might be a hormonal problem that could be fixed. I've known one other person who told me his first ejaculation was on his wedding night. They had several children. Thus, I would conclude that it is unusual, but no major concern if you are otherwise healthy.
No one is required to masturbate, but that doesn't mean the act (done without pornography or lustful thoughts) is sinful. Personal opinions do not determine whether something is sinful or not. Only God determines what is right or wrong.