Hi Never, 

Welcome, and I'm sorry you need to be here. 

Hopefully someone will jump in here who had to explain their new family situation to younger kids - mine were young adults when my GXH came out.  In the meantime, I'd like to separate out your questions in a way that might help. I don't know how old your kid is, so these are general ideas. 

Looks like your concerns are 
1. How to have the Where Do Babies Come From talk with your kid
2. How to include homosexuality in that talk
3. How to talk about your own specific family situation 
4. What happens if your child repeats what you say to your ex, and he gets angry. 

1. For The Talk, there are some really great books out there.  Try It's Not the Stork by Robie Harris.  It's the first of three books by the author that gradually ramp up in terms of information and detail.  It's an easy way to talk to your kids about sex.  One of her books has a short piece on what it is to be gay. 

A couple of notes on The Talk: generally speaking, it's more awkward for parents than it is for kids!  It's good to start by asking them what they know about where babies come from.  Most kids have some idea or have heard a little bit, but they haven't had the whole story put together, or they've put the pieces together in a way that doesn't quite fit. 

The nice thing about YOU having this conversation with them is that you become the authority, and an Askable Parent. 

2. For straight AND gay sex talks, keep it age appropriate.  Some online research will help you get some idea about how much information a kid might be able to absorb at a given age.  Part of this will be driven by what they say they know, and partly by the questions they ask you.  If you're not sure how to answer a question, you can always say that you don't know, but you'll find out and get back to them the next day. 
It's in your best interest to be the conveyor of accurate information - kids have access to incorrect information from friends' older sibs, TV, and the internet.  Preferable that they're not Googling their questions!  

3. Your own family situation.  Again, age appropriate is the key.  There are great children's books about divorce that might help you here.  If your kid is young, there's no need to assign blame or over-explain.  IMO it's not appropriate to tell your kid about the abuse until they're young adults.  I imagine the delicate question here is "why did you get married if Dad is gay", and I imagine a good answer is "because I didn't know he was gay".  If your kid is young, they might not ask, and if they do, they probably aren't looking for details.   

4.  If sex and divorce is explained in an even handed, age appropriate way, your ex might still get angry - but if your conversation with your kid doesn't assign blame or include a detailed psychoanalysis of why your ex was in the closet when you married,  he doesn't have much material to work with.   If he's volatile, you aren't going to be able to control what does and doesn't set him off. 

If it's within your resources to work with a therapist on this, I think it would help a lot!