Did Our Lord say to His disciplies or anyone else that He taught or preached to" Hey I know lets have adult talk and we can talk about sex issues" I don't think so
Well, we don't know. All we know is what is recorded.
Also, it may be helpful to clarify that MY thoughts are regarding general sex issues, and not specific, personal problems people are going through.
For example, if a woman finds out that her husband is cheating on her, I think it would probably be inappropriate (and maybe a bit dangerous) for that woman to share such personal information on the internet, unless she was doing so totally anonymously and in an attempt to get some Christian fellowship/counsel on what to do.
Besides prayer and the teachings of Jesus, Godly counsel is also a way to hear from God.
Also, the question itself, "what is the Christian response to adultery in a marriage"? Sounds like a reasonable discussion which could very well include sexual content.
What about "How many times should I date before getting married"? OR "Should I have sex before getting married to know if he/she really is the one I want?"
What about "Is masturbation really a sin and if so, why"? It's not only teenagers who struggle with that issue.
Or, how about what it takes to keep a married couple interested in one another? Paul talks about how a couple should not keep themselves from one another for extended periods of time to avoid temptation. That sounds sexual. He talked about virgins; that is sexual in content, too.
I know of workshops designed for married women, and workshops designed for couples on how to keep the marriage sexually exciting. It was a genuine attempt at helping women/couples to understand that discussion about sex doesn't have to be dirty or taboo, although discretion should always be used, too.
I think the idea for an adult forum, here, is something similar to that; to create an environment where Christians can discuss issues without feeling like they'll be laughed at, betrayed or whatever else may happen in their other environments.