It would certainly put my relationship in danger and that's enough of a reason for me to keep quiet
How would telling the OBS put your R in danger? The backlash from your cheating H?
Your relationship is already in danger with him cheating I'd say, but it's an individual decision.
I saw a lady whose H had cheated and when she told the OBS he said he thought it was over, as he'd found out 3 years earlier, but his wife and her H told him they were sorry and it would end.
They just took the affair underground.
She said she never forgave him for not saying anything, because in that time she had another child which she wouldn't have done had she known.
She was at risk of disease as her DH was having unprotected sex with the OW, who got pregnant during the affair, but terminated it as she didn't know who the father was.
I'd really like to know if my DH was such a snake and was putting my sexual health at risk.
That's 3 years she knew nothing about it and it had been going on for 2 years before that. Why be happy to live a lie. In that time they'd celebrated birthdays and anniversaries and other special occasions while he was putting himself inside another woman.
Very often people say there is no point in telling the BW ( usually the OW says this), because she won't leave him anyway.
Telling is not so they will leave, it's so that they have the truth and the knowledge to make an informed decision about their future.
Some people are scared telling will push their husband closer to the OW. If that's the case then he doesn't really want to be with you or he wants to cake eat.
I've come across this scenario and the cheating H says I would have stayed but now you've told her husband I'm leaving. Well he can bugger off is my response.
Chances are he was still cheating anyway, otherwise he should have been NC with the OW.
At the end of the day it's your choice. You may find you say nothing and as in one case I dealt with her DH and the OW each left to be together 8 years later. The BW then wished she'd said something earlier, so the OWs DH could have been aware and it may not have happened saving her 8 years wasted while the affair carried on.
Affairs thrive on the secrecy and the illicit sex, so exposure is a way to kill it. It's not as fun anymore when it's out of the bag. Two spouses knowing make it harder to meet up and sneak around.
I think BSs who thinks all contact ends on dday is very naive. Having been in the position of supporting OWs as well, they say the affair continues.
They will find way and if the OBS knows, the other cheater may well decide it's not worth the risk anymore.
Each to their own anyway.