Easiest girlfriend to get this with will be Denise, the first girl you can date after completing the mission 'Burning Desire' given to you by crash. You can just keep taking her to ten green bottles (the bar right outside grove street) and she should always enjoy it, this is also literally around the corner from her house. Something else I'd like to add, you can go to the LSPD showers and get a dildo there, save it and when you're on your date equip it, get close to Denise and you should be able to give it to her as a gift (a lil ribbon icon will show on your screen) to do this just press left on the D-Pad
, you can also give her flowers which can be found at the idlewood gas station. Doing this raises her like percentage (if I recall correctly by 5 percent) with you, which can be checked in stats under achievements, once it reaches around
40 percent, she will invite you inside for some 'hot coffee' after your date, accept it, get your groove on, and you will get the achievement. Do note that the first time you leave Los Santos for the storyline Denise pretty much breaks up with you, so I'd go for this chievo before you leave LS.
If you give her a gift each date, and take her to TGB or any place she enjoys it should only take 2-3 dates. If she isn't available for a date save at the grove safe house 6 hours will pass and she may be ready for one then, if not you can continue that step till she is.
If you think I made a mistake or want to add something to this solution feel free to comment below.