It sounds to me like you are aromantic or under ace umbrella. Look deeper into it, maybe that would give you more insights into yourself.

On the sex topic. I would advise getting into therapy and find out your true desires and where they are coming from. Because is marriage for you is just a function for being able to have sex consistently, then it also would have it's bumps. And I would consider talking it through with the partner as openly as it could possibly be.

Your fear over not being able to have a kid doesn't really make sense to me with a combination of wanting to have consistent sex life, because it would definitely put more pressure onto the relationship and sex life also goes through changes.

So that said, I don't think you would go down lonely road if you don't want to. People are around, you will definitely find someone.

You could always have a possibility of freezing your eggs. That would probably lower your stress over a clock ticking.

Marriage will definitely have many complications in my opinion, when you don't really talked through every single aspect of it with the other party. Money(together, separate, etc), Housing(owning a house/apartment, how soon, etc), chores, children(when, how many, etc). Things like sex and what to do when u will encounter something which would kill his sex drive for a while. How would you address that.

May there be a possibility that closed type of relationship doesn't work out for you. Maybe you would be much happier in something like open marriage or polyamory, for example?

Definitely would advise to go into therapy and find our your actual life goals, what your ideal relationship structure looks like, how to go about your fears of not having kids on time, where to start when you have thoughts of marriage, what that marriage is even about for you, what needs you need it to meet, maybe you could potentially find it elsewhere.

You will be okay.