You seem to be confusing what a Fade spirit may or may not be capable of with a mortal with what the Chantry teaches and therefore condones. The Chantry teaches that it was a spiritual marriage. The reason why it previously insisted upon its clerics being celibate was because they were meant to be mimicking the relationship with the Maker that Andraste had; so they are married to the Maker. Of course that overlooks the fact that Andraste was also married to a mortal and had a normal married relationship with him, including children. Which is why Leliana feels able to make changes in order that she can continue her relationship with her earthly lover at the same time as being Divine. There is still a clear distinction between the two types of relationship, though.
So far as people keeping concubines/mistresses/lovers is concerned at the same time as they are married, this seems to be accepted practice throughout Thedas, at least so far as men are concerned. Some societies require the person to be discrete about it; others are quite open. There is nothing actually in the Chant about sexual relations and therefore the Chantry doesn't get involved in the issue. In Orlais both Leliana and Vivienne are recognised as "official" mistresses of important nobles and Vivienne says that she had a perfectly amiable relationship with her lover's real wife. This seems to reflect the fact that most marriages, at least among the nobility, are arranged and politically motivated, having nothing to do with the feelings that exist between the spouses. So long as the required heir is produced, everyone seems happy. This is where Celene runs foul because she has maintained a lover without doing her duty, marrying appropriately and producing an heir. This is also where double standards are likely to apply, just as they did in the real world, when it comes to a married woman maintaining a male lover because of the doubts over the parentage over any potential offspring, whereas it is very clearly with a married man whether the child he has fathered is legitimate or illegitimate. It is all to do with matters of power and inheritance and thus any censure applied comes from the secular society they live in, not the Chantry.
So the Chantry does not support adultery but it does not condemn it either, because apparently the Maker has nothing to say on the matter.