You looked up the word 'unfaithful' the word we're looking for here is infidelity:
in·fi·del·i·ty ( P ) Pronunciation Key (nf-dl-t)
n. pl. in·fi·del·i·ties
Unfaithfulness to a sexual partner, especially a spouse.
An act of sexual unfaithfulness.
Lack of fidelity or loyalty.
Lack of religious belief.
You're really reaching here. You're misappropriating the definition of 'constant' - constant could mean doing the same thing every day, or constant could mean doing what you both want to every day (among other things).
If my girlfriend and I mutually decide to bring another woman or (
![Lips Sealed :-X](https://www.jedidefender.com/yabbse/Smileys/classic/lipsrsealed.gif)
) man into our bedroom, we're not engaging in any infidelity, not even close.