Hi. If a married woman sleeps with another man, but they have protected sex (ie: with a condom) is she then forbidden to her husband?
Yes she is forbidden to her husband. The reason is that although the condom blocks the sperm from reaching the male, regarding the relationship it is considered the same act.
This is besides the prohibition of a married woman having an extramarital relationship, which is a terrible sin, and in the time that there was a bais din both the man and the woman would be liable for a death penalty.
Nowadays, that our culture is very permissive, there is great temptation for us to act promiscuously, and it may be very difficult to overcome these temptations. We have to remember that the master of the universe, will pay us back with a lot of more pleasure for keeping his commandments, and the more difficult it is, the greater the reward will be.