[MM] Nearly had sex with my best friend

Hello Reddit! This is my first post, so please excuse any dumb mistakes. I am a bi-sexual male who is introverted and aromantic. I have never been in a relationship, dated or even kissed anyone despite being 24 years old and having gone to (and graduated from) college. I wasn’t a late bloomer or ugly, I just never had the need to be in a relationship with someone. About four months ago I decided I needed to be more active and meet new people so I signed up for a gym membership and started to take classes. The trainer, Tom, is a fairly attractive guy who is 5’6 and 170Lbs with blue eyes, brown hair, fair skin, scruff, and is in much better shape than I am. I am 6’2, 135Lbs with brown eyes, brown hair, fair skin and high cheek bones. I am self-conscience about being underweight so I go to the gym every day and take classes from Tom. After a few months we became friends and grew pretty close. I found out he had a girlfriend in a different city that he visits every weekend and he found out that I was about to go into the Navy as an officer which (to my surprise) made him upset. I thought: “geez, does this guy really care for me that much?” I’m not going to lie, there was some sexual tension between us from the get-go, but it really surprised me.