Generally very easy. You have the option to "friendzone" any girl, meaning no further erotic will happen. Friendzone everybody but your true love, and you have plain sailing.
You will need to advance some of the girls' storylines to get others unlocked though, but that doesn't need to involve youknowwhat.
That being said, it is a bit more complicated with Emma and Samantha. Everything is more complicated with Emma. You cannot totally avoid Samantha when going for Emma (though I might be missing a path here, she has multiple ones). However if it would help your conscience, anything with Sam would happen before you even meet Emma, so there would be no cheating on your love whatsoever.