my impression (which MAY completely be wrong) is that she was having sex with married men because they were safe meaning that she just wanted sex without the commitment and married men wouldn't call her up and expect her to date them (well...I mean...take everything there with a grain of salt as married men are also not supposed to be having sex with strange women in bars)
so you might say that sex is just a physical thing right now but clearly it isn't or she would be having random sex with other people
(look at sex like running...i mean...so she likes to run, she runs indiscriminantly and then you come along and she stops running except to run with you....so she likes running with you...maybe hoping you will marathon train with her...)
saying you aren't running a marathon but running with her everyday kind of gives the wrong message
and the idea that you can save her from herself or her son is a little demeaning...maybe she doesn't want to be saved maybe she doesn't feel she needs to be saved
just because you don't approve of her lifestyle doesn't mean she needs to change it to be healthy... maybe healthy for you