1. I am a middle aged man. I would have no problem with this. I don't think your "inexperience" will be an issue with most men.
2. In any serious relationship, communication is very important. If a woman told me that she had never had sex, and that she wanted to take it slow... I would be understanding especially if I saw a future in the relationship.
3. I don't think you should mention this on a dating site. This is something you can talk about after a few dates with someone you want a relationship. The first few dates are generally a time to get to know each other and to see if there is chemistry. After a few dates... if it is going well, you can talk to him and say "I like this relationship and I am serious... I want to talk about this...".
This probably won't be as much of a big deal as you think.
4. It really helps me, as a man, when I know for sure what my partner needs and wants. My advice would be for you to do something thinking about this on your own. What would a sexual relationship mean for you? What do you want in a partner? What do you need to feel comfortable.
If my partner can communicate these things clearly to me, it makes it much easier for me and it also helps me to be patient, respectful and caring.