I don't know how in the world you drew that conclusion from what I said. I see the situation you present - having sex with your wife only - as following God's Law.
Any occurrence can be termed a "situation", so the term could be used to define that, but my use of "situational ethics" refers to adapting the rules to fit the situation - like, I can only have sex with my wife, but I'm marooned on a desert island with a woman not my wife, possibly forever, so that will be OK for this situation now.
I stand by my use of the term for what @David put forward. He stated that any watching of porn is sin. Period. He wasn't ambiguous about it. He also stated (at that stage of the threads) that my making a video of me and my spouse in the privacy of our home is porn, by virtue I suppose of some definition that he applies to porn. But watching it, "the porn we made", if it is only the two of us watching it, is not sin, but rather OK. That's changing his position to fit the situation, which is "situational ethics" to my way of understanding that term.
If @David had said, no, your making that home video is porn and watching it is porn, and thereby, is sin, (the title of his thread was "Can watching porn ever NOT be sinful" - my take on those words to mean that he is stating that it always is) I wouldn't have agreed with him, necessarily, but I would have respected him for standing on his principle.