Please, just don't do it.
She is already worried and anxious. If you want to play together and be intimate, do so, but keep your husband out of it. It's like taking a tenuous situation that could possibly work with oodles of tlc and gentleness and instead adding an accelerant to it which will run wild and destroy everything. So your choice is to keep gently reassuring her that you both being intimate does not bother your husband and that if the sexual relationship doesn't work between you two than you can you will both happily continue with a platonic friendship - WIN WIN or you can invite your husband into it, introducing a brand new unpredictable dyad to the mix vastly over complicating all your lives which will more than likely implode leaving destruction in its wake. LOSE LOSE.