Witchipedia

Usually the "love" spell gets them wanting your body, and then the rest is up to you. If you've already got him in bed, then you've already accomplished the goal of most "love" spells. So… I really don't know what more you're asking for at this point, but I'll try to elucidate in general and maybe I'll land on something that's helpful to you.

The act of engaging in sexual activity causes the body to release hormones that act something like heroine that can cause an addiction-like response to the person you are having sex with. This response is often (very, very often) mis-labelled as "love". Some people are more susceptible to this than others, as in all things. This is the real reason you should not have sex with someone you do not trust, because you run the risk of giving them this power over you. With a little forethought, self-control and a certain lack of empathy, you could use this knowledge to gain control over someone else too. Just because you don't know how it works, doesn't mean it's not magick.

The use of a bodily fluid in the spell provides a sympathetic link with the individual, since you are both there already, you don't need sympathetic links. Menstrual blood is used as a sympathetic ingredient in some love spells, that is primarily because it is a intimate body fluid that is easily collected when the timing is right and bodily fluids are often incorporated into spells. You could also use your vaginal secretions in between cycles and it serves the same purpose, they simply aren't as easy to collect in quantity. If you're sharing bodily fluids already the presence of menstrual blood isn't going to make a difference.

PS Use a condom. Condoms save lives.