I have seen it cut both ways. Some girls do get into boys and forget horses. Likewise girls get into soccer, belly dancing or music and forget horses. I think there is an element of horses are either in your DNA or not. You can expose a kid to them but you can't make a kid love them or want to own them. All three kids in my family grew up around horses and all of us did some form of 4H as children. Of the three of us only one (myself) is still involved in or interested in horses. Thats okay, my siblings are wonderful people who happen to have interests outside of horses. I ended up taking a break from horses for about a year maybe two and then found a way back to it. In the end, that is how you know you love something, you spend time apart from it and manage to find a way back to it.
Dating wise, I don't consider it a requirement for the guy I date to be into horses. They need to tolerate horses but they don't need to ride them, or be into them. They should listen to me talk about it a little bit with the same respect that I give them when they discuss something they are interested in. If the choice was ever presented to choose between the boyfriend and the horse (or the dog, cat, or my family) than the horse would win. Mostly because if someone you love is making you choose between them and something else that is really important to you, than its not really love (at least not the kind of live I want).