I think you have a few things going on that are all getting mixed up together.
1. You are feeling guilty that you couldn't protect your child from sexual abuse.
2. You are struggling with him being gay and kind of wondering if it had anything to do with abuse.
3. You don't want him to get hurt again and are unsure about how to protect him.
It's clear that you love your son and have been thinking about this a lot. I think those two facts are going to go a long way in making this better.
We can't protect our kids from everything- actually hardly anything once they make their own decisions. If you did your best to be vigilant and dealt with the abuse as soon as you knew... then you have no reason to feel guilty. Grief for what your son went through- yes. Guilt that you didn't protect him- no.
In your heart you know your son didn't become gay because of the abuse (you mostly acknowledged that). Some statistics say that 1 in 20 boys are sexually abused- if abuse caused them to be gay, there would be waaaaay more gay men in the world. Your son is going to be fine because his dad loves him enough to try to understand him.
Your worry about him getting hurt is normal parent fear about our kids getting hurt- only it's not hypothetical for you. Your son has been hurt and you are maybe overly-vigilant because you don't want to miss something that might be a danger to him.
Tell him all of this. Tell him you are scared for him- and more so because of the abuse. Don't be afraid to talk to him about how the abuse he suffered makes you scared to send him out into the world. Tell him that you are doing your best. That you trust him to take care of himself. That you'll always be there, just off stage, to help him out.