Hmm... sounds to me like it could be low testosterone. If you hardly ever feel the urge to masturbate then this seems quite likely. See a doctor about it.
Could possibly be low testosterone, if so, you can take ZMA ..not that expensive.. since you are building muscle rather ok, maybe it isnt testosterone as the main issue.. but it might be worth it to do endocrinology fertility test .. FSH, LH, TESTO PRL are hormones that are usually tested...in rare instances, there are cases where some ppl had genetic mutation that affect the "switches" which influences puberty. Some had mild mutation, some had serious mutation. The serious mutation could either be causing TOO active or TOO low activity of the affected switches. When it is too active, the puberty come too quicky (eg 2yr old and testo is at adult level and penis is enlarged). When it is too low, then puberty might not come.. mild mutation causes puberty to be delayed. So, this sort of thing, if it is relevant, requires genetic testing. Genetics of Puberty isnt yet completly resolved; ppl are just finding the genes rather recently that can influence puberty