Feeling that ownership after a casual sexual encounter is a lot easier than feeling it day after day in a long term R... through arguments, illnesses, dirty underwear, tantrums, ED episodes, weight gains, and the other hazards of being with anyone for a long period of time.
Sure when you give it one time or a few times to a guy, you feel certain of your ownership... but do you still feel that certainty after years with the same guy? Especially if he starts to feel "entitled" to it. (I realize that's not the problem in your sitch, but it is in some here.)
It's like if you loan your lawnmower to your neighbor a time or two, you know it's still yours. But if it sits unused in his garage year after year, whose is it really? (Okay, bad analogy.)
