If you don't want to be monogamous and that is the basis of your problems, then certainly be clear and honest. But if you want commitment then you can work on the sex life with him. Otherwise it is just an excuse to be with someone else. So you need to make a decision and stick with it. You may have to accept that he has a low sex drive and is not adventurous. If you remain with him you have to respect the boundaries of this relationship. You can do as a couple something he is not comfortable with so if that means sex once a week in the bed, then that's the way it is. If you are truly not going to be happy with this, then seek outside experiences but don't lie about them. Cheating is anything that he would not allow or encourage. If he doesn't want a threesome and you do it with 2 guys behind his back then that's cheating.
You can investigate more and varied way to spice things up but not so much where he is uncomfortable. Anxiety is not productive to his sexual prowess.
Try to figure out what you want, do it in a respectful way, and stick with it. Sometimes you can't have everything in one guy. But introduce ideas that are not so scary