How to set boundaries politely when I know the relationship moving forward isn't doing either of us any good?

Anonymous
So my friend M (30) has been in a long term, long distance relationship and I F (22) just got out of a long term relationship. We have been friends for a few months and as we have A LOT of likes and interests in common, we started connecting really well. Slowly I could notice that he started getting flirty and romantic with me. Daily texts started becoming calls, video calls and even late night calls that go on for hours. I really like this guy but I don't really reciprocate romantically as I know he has a girlfriend and I don't intend to get into a relationship anytime soon. He keeps saying that he regrets not having met me earlier and there are times where the conversations also get intimate and I just don't go along but I don't know how to politely set a boundary. I have never met someone this similar to me and that makes it harder for me to see things clearly as my head doesn't want to imagine him being removed from my life. I really love hanging out, talking and traveling around with him but I don't want to get stuck in something that turns into an affair because I didn't set boundaries. Would really appreciate insights on setting boundaries while not offending him.
How to set boundaries politely when I know the relationship moving forward isn't doing either of us any good?
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