I thought the 5th house signified a casual relationship too....so a relationship at the beginning stages where you have not made a full commitment to each other, not living together etc.
There seems to be a very thin line between the 5th and the 7th and of when to cross it.
I think you will find some differences of opinion over this. John Crawley would say the seventh house is the person, unlesds their relationship to the current is clearly defined (my father, my brother, my daughter, my friend: in which case they get the fourth, third, fifth, eleventh respectively - this list doesn't cover all possible relationships. What you and the other person do together belongs to another house - play bridge (5th), hunt for buried treasure (4th).
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This distinction seems clear but in practice it isn't. If the question refers to my bridge partner, I wouldn't use the seventh for the person, as their relationship to the querent is defined by what they do together. I'd use the fifth for that person.
Now in relationships there's something of a grey area. This comes from the occasional difficulty of establishing what the relationship to the querent is.
Lee Lehman settles the distinction between fifth and seventh in terms of relationships by asking the querent 'Are you living together?'. If the answer is yes, then the relationship is seventh house, if not (and it's a sexual relationship) she takes the other party as fifth house.
But I don't agree with the distinction being so easily resolved. Clearly the question 'Will I marry X?' relates to a future event and whilst that, if it comes about, will involve them in living together, it does not have to be the case that they must be be living together at the time of the question.
However there must be no obvious obstacle to a seventh house long term relationship. In your question the other party is married, so a 'marriage' is not legally possible in the current circumstances. If he left his wife and moved in with you, then he would establish a
de facto seventh house relationship as per Lehman. If he announced divorce proceedings and moved out of the marital home into his own accomodation, I'd treat him as seventh house, as he is taking steps to remove the impediment and opening the possibility to a 'marriage'.
If you had asked, 'Will he move in with me?' or 'Will he divorce his wife?'; I would treat him as seventh because either of those actions would change the nature o your current relationship to a seventh house one.
Note that the exact framing of the question can change the meaning in subtle ways. Bad phrasing of the question can give you an answer you never expected and prevent the chart giving you the answer you seek.
