Posting this on behalf of a friend (no, really) who isn't a MNer but will be watching the thread. She is single and has a DS, she works with a man who has a partner of several years and his partner has a child who lives with them.
Soon after my friend got this job, she and this man began texting each other every evening after work and the level of flirting in work and via text message continued to escalate. It's now at the point where he moans about his partner (the usual script: she doesn't have sex with him, they don't really talk to each other etc), he sends smutty texts making reference to sex, and he's told my friend that he likes her but 'it's complicated'. As far as I'm aware, nothing physical has happened yet.
I've told my friend that she needs to disengage from this now, but her response is that she knows it's awful but she can't help herself because she really likes him. She knows the damage that she and this man are potentially causing to his partner and her child who is presumably pretty attached to him seeing as he has a parenting role and has done for years. But this knowledge appears to be overridden by the flattery of this constant pinging of text messages every evening.
I know it's really none of my business, but I want to try to help my friend to reflect upon why she would want this for herself, and why she would compromise her morals by being part of doing this to a woman and child (though I accept responsibility lies with him).