I agree on arlandonblooms comment. The more you and your boyfriend have sex the looser you become also try lots of lube.
I ride too! Well I used to and then I stopped for a while because my parents couldn't afford it but im trying to get back into it I've ridden dressage and hunter jumper and I've ran barrels before too. Ive been riding for 8 years. Where do you ride?!
First of all, don't stop doing something because of sex... That's absurd. I'm pretty sure the only thing you should do to make the muscles not so tight is to keep on having sex, try different positions?