I would never do such a thing behind anyones back. And if the couple did breakup, I would ensure my friend had no problems with my seeing his ex if I happened to be attracted to her and she to me. My friends, most of which are actually female anyway, are all very aware of my unique endowment and are all actually VERY protective of me. Like true friends, they are the ones who will give me the most "grief" and also seldom pass on an opportunity, situation permitting, to try to cause me a bit of embarrassement or a bit of awkwardness now and then. They mean no harm, just what are friends for if not except to give us grief and occassionally embarrass us?
But I also know how protective they are of me. In public places, if someone is starting to make a scene by staring, pointing or commenting too loudly, they will often step up and run interference by confronting the person and shifting the attention. And my friends, especially my female ones, know my "type" and what attracts me, and what type of woman might be compatible both mentally and physically. Most of my female friends have been women I once dated briefly and while not sexually compatible with me, they realized it was not anyones "fault"...and were mature enough to want to stay close to me as a person while realizing our anatomies made anything more than friendship not practical.
I have had some 3sums with some friends who were couples, but that was at their great insistance and usually with some cajoling on their part over time. That has been rare. Just because a male friend may be attracted to his wife/girlfriend/signicant other...does not ensure that I will be as well and interested in her in any way.
However, all that said. When I was in my very early twenties I was dating a woman and it was just not working out. She was very possessive and while not physically able to take much, she could do a few inches and was in "lust" and also a very needy/clingy person. I tried to break it off repeatedly but I was not very good at it and she was the tearful type and very clingy so we stayed together and it was not a good experience. Finally, I was downtown alone in a gasthaus (pub) one Sunday afternoon and her best friend walked in. Her best friend was curious and had actually seen me erect because of once having walked in on her friend and I. When my "gf's" friend walked in I got the less than bright idea that if I pretended to hit on her friend, the friend would play me off for the reason she knew she could not handle me, but would also run straight to my "girlfriend" and tell her what I had done by making a pass at her. I made the pass, she politely declined, left the place within ten minutes or so, and by the time I got back to my place later that afternoon my girlfriend was in tears (a bad side effect of my plan) but was breaking up with me. Fortunately we were not living together at the time. My plan worked, but I really felt bad about it but other efforts to get out of the relationship had failed. Life is a learning experience and I learned that possessive, irrational, and clingy was not my type.
It happens that I often get a lot of offers from people I have just met or recently met, almost all based upon my size and nothing else, but I am very selective and my sexuality is not a "free-for-all". And besides being selective on traits like personality and intelligence and visual appeal (the lust factor) there must be some minimal sexual physical compatibility or I know any relationship would not be right for me.