It's just kind of a self drag really, your sleeping with someone who likes you enough to sleep with you but not enough to end their relationship with someone else
It is also super tragic that this seems to be a pattern for you. Btw you are NOT helping the other person by "proving" anything, if that was your goal you would just show him/her your texts and possibly a recorded conversation, instead you word it differently so you don't actually mention it to the other person but have another reason to justify it.
If that is what you like to do cool, don't try and justify it and don't treat it as a bragging right all of that just screams "i feel guilty" tbh