An interesting assemblage of questions, will do my best to answer them.
Ahh griffons... they are certainly a monster race I highly admire. Morrigan does as well, has even mentioned offhand that she wouldn't mind sharing if one happened to fall for me.
Popeatine wrote:What would Morrigan had done if you had just accepted one of the monsters you were powerful enough to avoid or just offer yourself to them?
Would she had abandoned pretenses and just step in and take you?
I asked her this once and she just smirked and said she would have stepped in and ravished us both so silly that even if the other monster stayed interested in me Morrigan would have been the dominant wife. She may have been kidding though, as Morrigan isn't usually one to easily share things she wants. It would have probably depended on just how much the other monster wanted me and how much I wanted her.
How is life married to a Griffon?
In general who wears the pants in the relationship, the husband or her?
Both races may take offense to this, but there are a lot of similarities to "untamed" dragons when it comes to married life. Griffons are open and daring creatures, never afraid to voice their thoughts so they are less tsundere than dragons but have similar pride in themselves and a tendency to be dominant in the relationship.
How exactly are Griffons able to fight Dragons?
It comes down to speed and sheer ferocity. Griffons are very strong of course but they are not as strong or durable as a mature dragon. They are, however, very fast and agile, with talons capable of cutting through all but the toughest dragon hide. No one attack from a griffon would likely do much more than annoy a dragon, but dozens of slashes and constant harrying that the dragon can't properly counter would conceivably lead to most dragons deciding the griffon's treasures were not worth the effort.
Its also worth considering that griffons aren't against living in groups and working together to drive off powerful foes.
Are Griffons jealous? Would they be willing to let their husbands form harems?
They are very proud and enjoy having their husband's desire focused on them as much as possible, so a harem would likely be seen as a threat to that at least at first. But I wouldn't say they are anywhere as jealous as monsters like lamias and such. The few griffons I've seen that were part of a harem though tended to be small harems, and the griffon herself was either the top wife or shared that position with another wife.
Are Griffons even more protective of their husbands than Hellhounds?
They are both guardian races appointed by gods, so its probably safest to say they are roughly equal to each other in that regard.
What is more dangerous, attacking a Hellhound's or a Griffon's husband?
See the above, I'd hesitate to say if either is more dangerous.
What would a Griffon do if her husband wants to take the treasure, would she let him?
I'm not aware of accounts of this ever happening, beyond the couple potentially using some of the treasure if they needed something. That is because a natural part of becoming the husband of a griffon is that his desire turns from the treasure to the griffon instead.
If a Griffon is having sex with her husband, could another thief use that chance to sneak in and steal the treasure?
Potentially, given her senses will all be focused on stoking as much desire from her husband as possible. But as I noted above, its not uncommon for there to be more than one griffon guarding a particular treasure or ruin. One also has to consider that the couple may have children who have taken over guard duties.
Would it be possible to approach a Griffon's ruins with such a sheer greed that the Griffon gets incapacitated before she can attack out of sheer lust?
Feelings of lust on their own can never incapacitate a monster as far as I know.
Since Griffons can sense emotions, does that mean that her husband will never be able to lie her?
Griffons don't sense emotions in general so much as strong desires and the direction of those desires. Thus the ability to get away with a lie would depend on the lie and the circumstances of said lie.