So you had sex with them ? How long have you been dating them. I would say stop being romantic ( do not even touch them ) immediately and be honest with your intentions. These girls are not going to be sex buddies and you are targeting wrong girls. Probably using some manipulation tactics knowing or unknowing to sleep with them.
If you are using some apps to get these girls then as someone said let her know you are into relationships. You can directly or indirectly say you have slept with x amount of girls but have not found the one or something like that. Do not have to be complete jerk but you must show your sexual, playboy intent. You can still get laid but own yourself that you are a cool guy who is not needy and has abundance. No bf vibes or manipulation is needed. I think for casual sex night game is probably best, so find venues in your area and see how it goes. I may be wrong but girls who like to party a lot, drink, smoke have tattoos, piercing are probably ideal target for you ( I know many such girl are trash lol )