Perfectly normal to worry about and feel self-conscience about your writing. It's bound to happen because 1) you care about the story you are writing and 2) there is an element of vulnerability. Try not too stress too much, people are generally supportive on here. Writing is difficult after all, just try to enjoy the process of writing and learning. (none of us are going to make a living out of it on here so we do it for the enjoyment)
Experiences can help by giving the story an authentic quality, however, I think you might be placing too much emphasis on it. Tolkien didn't have much experience with dwarfs, elves and a giant burning eye at the top of a tower (at least I hope not - maybe he secretly did a whole lot of acid?) and yet that story is loved by millions.
Plus, sex is only part of erotica, the depth of the relationships, or the lack of, (ie cheating/one-night stand) is where a lot of the story is found. Even if you are inexperienced with sex, I'm sure you have been attracted to someone, felt nervous approaching them, hoped they would look your way etc. So you do have plenty of relevant experience for the stories you write. As for the areas you're less familiar with, reading other people's work can help with that. The more you write, the more you'll learn what works for you.
The experience which we all need to write good stories, is and always will be, writing experience. The more we write the better we become, and the more able we are to capture the mood and feelings of the characters.
There's countless more experienced and far more talented writers than me on here, but that's my thoughts. Hope it helped and please don't get discouraged from writing if that's what you enjoy doing.