@Stars44
The way i see this issue of yours,
is you are obviously in a coercive co dependency emotionally psychologically sexually abusive relantship,
there is something about your husband that he is master manipulativor,
hence the threesome,
I think doing something you regret whilst out on a night out, was a moment of weakness on your part which highlights,
the lack of "emotional intimacy" in your relationship with your husband,
with your husband sex, is like a bargaining tool a weapon that he uses to manipulate you, to gain the upper hand,
as he " gets off on the drama and the tension the manipulative control" this creates,
he probably finds normality too boring vanilla,
Hence the need to instigate high drama ect to make life more interesting 🤔 every so often in whatever way he thinks is most effective at that time,
Your husband is using your son taking advantage of him, cause he is impressionable teenager as a manipulative way,
so by causing conflict it potentially undermines "togetherness creates tension and disharmony",
so your son is in environment mixed loyalties and having to choose who to be most loyal too,
Hence your husband lying about you having a affair,
Please 🙏 Don't listen to @pinkyredrose advice,
you don't need to admit to your son about the coercive manipulative control threesome you had,
I think you need to chat to your son and explain to him that you no longer love his father because of how abusive he is in so many different ways,
State clearly you are definitely not having an affair,
but you just can not simply live with his father any longer
Cause their is no love or respect for you in this relationship and its been like that for some time or quite a while,