I believe I can relate, at least a little: my husband is not into costuming the way I am. Over the years, we've tried out each others' favorite pastimes and there hasn't been a spark. In the long view, though, we both know that a marriage is a connection, and it's about sharing things.
I learned to play his card game, to the point where nice people say I'm good at it. We play every friday. He listens very attentively to my costuming issues, triumphs, and troubles, and even tries to suggest solutions to my troubles (he's very analytical, so it's more of a gift than it sounds). He doesn't fake interest in the subject, but always shows interest in me.
Costuming is who I am. I tried to deny it for a while, but it just comes out anyway. He accepts it, even if he doesn't love it. He supports me because he wants the bonds of our marriage to stay healthy, as I do.
One of my other pastimes was the SCA (historical recreation). It also left my husband bored and cold. I simply don't do it any more, because I'd rather spend our time together enjoying something together. I still read SCA forums and look at SCA pictures, but I have found other substitutes for our time that I also enjoy.
I spend a lot of time on this forum talking costumes, and I spend my time while he is at working either working on my project designs, or helping others with their costumes. I get my "fix" by feeding it on my own time, but also by occasionally offering my husband opportunities to get involved with it too. And when he doesn't take me up on it, I remember that it's not that he doesn't love me, it's that he doesn't love costuming.
I'm trying to say that you can have a relationship with someone who isn't into this, if a) they accept you for you are, b) you work to form other bonds doing things you both enjoy, c) you both make concessions. Without these three things, relationships tend to wither due to frustration, resentment, mistrust, and disappointment.
If your SO can't treat your cosplay with respect, then your SO isn't respecting you. Talk to him/her and explain what you feel, what you need, and what you want from him/her. It is up to your SO to respond with "Okay I can accept that" or "I'll try to see it in a new light" or "Your cosplay embarrasses me and I hate it". Be respectful of the answer: if it's "I'll try", change is hard and it takes time and patience. If it's "this is stupid," then it's time to decide what you want more, your SO or your cosplay. I tried living without costuming and failed; I live fine without the SCA because I don't need it to be whole and happy.
Please don't be with someone who looks down on you; you deserve better and you can do better. Your SO doesn't have to respect the cosplay, but he/she needs to respect YOU.