How can I let my family know that I'm not interested in talking about my sexuality?

I'm 19 and gay (if this is relevant). I guess I don't really care about my family finding out I'm gay, but I've definitely been around and I don't really want my family finding out about that. Is this normal?

My dad once asked me about my sex life, and which sex I felt more attracted to as some father-son bonding thing. I danced around that question, and tried telling him I wasn't really comfortable talking about it, but he persisted for a while but eventually stopped.

Then my sister asked if I ever get horny. Then she told me she gets horny all the time. I told her I didn't want to talk about this, and she acted all surprised and claimed that I "don't trust her" and that I'm hiding something.

I don't know why, but my family is just acting like I should be so comfortable talking about my sexual business with them and I'm not. It's not just because I'm gay either, if I were straight I'd still feel awkward. But they act like I'm hiding something and I need help.