❓❓ What to do when another man wants you..❓❓

This is a very unusual situation for me, having been in a long term relationship. I have been friends with a peer at college since orientation freshman year, and it's been a very professional relationship and he has never flirted or hinted at anything more than being a friend to me. He texted me, asking about an assignment, like usual, and he ended up by sending me a dick pic, me freaking out and deleting it, yelling at him, and he continued to send me dirty messages such as "I'm getting big an hard just thinkin about you" and "you don't know how bad I want to f*ck you right now". I proceeded to ask if He was drunk or high, but none of his words were misspelled and he preceded to tell me he was not.  Up until recently he was in a long term relationship as well- in fact, I'm not entirely sure they ever broke up. I told my SO the next day, asking him to look at the messages. He was POed to say the least, but offered me little advice, and after a while didn't seem to care about what this friend had said to me. Now this is a man who is naturally possessive of me, so when a situation like the arises, shouldn't he be more so? He finally talked with him, and instead of giving him what's coming to him, all he really told him was don't do it again. Ladies, is this a bad sign when your man isn't offended by a situation like this? Or defensive of u? How am I supposed to handle the situation with this "friend"? And his gf? Has anyone been in a similar situation or have advice?