Well, nice of you to try to let people down gently, but if your way of doing that is to make excuses (instead of saying no), u run the risk he’ll believe you instead of inferring the no (which you don’t actually need an excuse for). A man’ll usually try to overcome an obstacle rather than give up IF he thinks it’s real/possible. Remember, he’s not taking no for an answer if you haven’t actually said it & u can always fall back on directness if subtlety doesn’t work
First of all, responding to you saying you have a boyfriend with “he’s not forever” is a very shitty response, no matter how you frame it, but second of all, a lot of us guys are dense when it comes to these things and take what you say at face value, rather than picking up on hints that you might not be interested. You can let people down easy without unnecessarily hurting feelings while still being clear and direct. For instance, you could say “I’m sorry, but that’s not something I want.”
*disclaimer, I don’t know the deets of every time you’ve done this so ik this isn’t applicable to every case (such as where you’re directly turning someone down). But for example “I have a boyfriend” is an example of something that may be seen as an obstacle rather than a dealbreaker, since he doesn’t know for sure if you’re faithful