my 'skewed' view on this is based on my own ruleset so may not be 'moral' per sa

If chatting to a girl and she makes it known she has a boyfriend to me, i wont chase her BUT if she then chases me i will fuck her. my flaky reasoning is that SHE has chosen to persue me and if it wasnt me she was fucking it would be somone else.
I draw the line at playing somone with a partner and using my 'charms' *cough* to get in her head and have her see me as new boyfriend material.

If i am with somone full stop who i can see is showing genuine feelings for me and i dont feel the same way and see an end point to whats happening--then i`ll have an honest chat with her about whats going to happen and or i`ll walk away - i dont like purposely hurting somones feelings.