That you do not believe in god makes it harder to comprehend your position on threesomes. People who belong to organized religions and regard certain texts as sacred would refer to something like "god created a man and a woman so that... " etc. etc. to justify the view that, say, attractions are allowed only to opposite sex and sexual encounters are only allowed between two people. You are not claiming that, and, therefore, your view that any sexual encounter that involves more than two people is irresponsible is especially arbitrary.

Also, to the extent that verbal exchanges can be described as intimate, you were having a verbal threesome with the wife and her gf when you signed up for the app that delivers three way texting (whatever that app is). So, in your mind, something is so special to sex that a verbal threesome is OK but a sexual threesome is not, right? What is it?