Why do I feel so guilty for this?

This guy I'm sure involved with, we sleep with each other but it's not a definite relationship. He's very physically affectionate toward me. I feel if there is no title then maybe I should be able to talk to other guys. I have been talking to a guy and the first guy doesn't know it. I really like the first guy but since he hasn't put a title on us I feel like I need to have a guy to fall back on in case he finds someone else. The issue is I feel guilty doing this and would much rather just have a relationship with the first guy. Do you think it would even matter to him if I just left him and purses the guy I'm talking to?
Why do I feel so guilty for this?
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