Boyfriend is transgender?

So my boyfriend recently came out to me as transgender. He identifies as a female and wants to begin the transition process. I love him/her (I'm sorry if I offend anyone but I'm not very framiliar with the transgender community and this is so new to me) so much but I'm very much a straight woman. I just can't see myself with another woman. We've been together for nearly 3 years and I was hoping to marry my boyfriend and have a family with him. Now I know we could still do that (he still wants to be with me) but I just don't see myself with another woman. I've got no issues with same sex couples but I'm personally attracted to men. I want to be there for him during this transition but I don't know if I can. He gave me some time to think about it and I haven't spoken to him in a few days. I'm taking this very hard. My heart is just broken and I feel like my life and the future that I could see with him is just shattered. And I know this isn't about me- it's about his transition but I just don't know how to deal with it. Can someone give me some advice please? Has anyone ever been through this?