My (35M) ex-GF (31F) walked in on me and my GF (25F) having sex and watched us, she then wrote a detailed email to our custody lawyers about the "sick" sex she witnessed. Where the fuck do I go from here?

DISCLAIMER: I'M DETAILING A SEXUAL ENCOUNTER WITH MY GIRLFRIEND, ITS ALL VERY RELEVANT

Backstory: my ex and I were college sweethearts but never married, we have a 3-year-old son who we share custody of. We broke up before he was 6 months old. I've been with my current GF (hopefully soon to be fianceé) for almost 2 years.

2 weeks ago my ex came to my house unannounced to drop something off for our son, she came by at 12:30 am, long after my son went to sleep. For emergencies, she has a key to my house and I have one for hers. My son was up on the 2nd floor and my GF and I were down in my office/den area. Apparently, when my ex came down from our son's room she heard us a little and according to her came to say "Hello", well she got to the hall and saw us; more specifically saw me laying on the floor with my GF sitting on my face and of course me eating her out. I seriously don't know my ex stayed to watch the rest. In all truthfulness my GF and I are toying around with the idea of having a baby, her anti-depressants make it harder for her to get pregnant so we're having a lot of sex and even before we've always been a little "dirty". We live in a home that overlooks the Pacific Ocean and we don't have neighbors who can look inside can look inside so we love to make love in front of the window.

My ex stayed for the whole time apparently because 2 days later she wrote an email to me, my GF, my lawyer, and her lawyer stating she can't allow our son to live in a house with "perverts" who behave like us. In the email, she admitted to catching us and staying to watch our "disgusting" sex and detailed out everything

Our oral sex which she said turned in to me "orally assaulting" my GFs "bottom"
The fact we had a sex up-against at the window with my girlfriend's naked body was pressed against the window
Towards the end of intercourse, I announced I was "close" so my GF said "Yes, yes, put a baby in me"
After finished we laid on the couch and I apparently fingered her and in my ex's words "fed her my sperm"

My ex now wants me to agree to give her "full custody" of our son because according to her it was reckless and perverted to have sex like that if he's in the house, she called my GF "sick in the head". I stressed that she knows our son sleeps through the night so he wasn't going to hear. She thinks I'm an irresponsible parent who was "neglecting" our pure and innocent son so I can have perverted sex with my girlfriend

Well, its been 2 weeks and my girlfriend is still embarrassed, and upset. She's ovulating this week and we normally would be trying but my son is here so she's afraid too.

What the fuck am I supposed to do?!?

TLDR; Ex-GF/Baby Mama caught my GF and I having sex and feels we're "perverts" and I'm a bad dad.