Unfortunately, you can't force your boyfriend to have sex with you.
If he doesn't want to have sex with you during your period, it's not going to happen. Regardless of what your flow is, he's just not interested. You need to have a conversation with him about it. Tell him that you're offended and hurt that he doesn't find you attractive during that time of the month. Ask him if there is anything you can do to make him more comfortable. Hopefully he'll be honest and you both can figure out some ways around his apprehension.
That said, if he's still reluctant to have sex during your period and you continue to think it's a big deal, then you need to consider a change. Sex is, of course, an incredibly important part of any relationship, and if the sex isn't satisfying you, then you should think about moving on. Sexual frustration, if gone untreated, can doom a relationship. And it might be best to end it now before you grow to completely resent him.
Ladies, is this a problem you've had to deal with?