If it's just looking at pictures or getting your rocks off visiting sites like this, then it's certainly not cheating, it's just porn and most partners accept such material is out there and is used for sexual arousal.
They might even hope to be the beneficiary of any resulting excitement, but obviously the difficulty with this type of porn is that it would raise serious questions in a girlfriend's head about how sexually compatible you really are.
So no it's not cheating unless you actually meet up with a sessionette and that is clearly a very sexual encounter, even if no sex or release transpires.
However, it's such an unconventional fetish, that I'm not sure cheating is even the right word, though a partner who found out would feel deceived at best and wonder who she was really with and what he really wanted.
Cheating is if she finds you with another woman whom she regarded as one of her sexual peers and competitors, ie roughly similiar to her or somebody more attractive whom she would be jealous of.
But an fbb is such a completely different type of sexual partner, and so unlike the body and lifestyle that most women aspire to, that I'm not sure they would see her as a direct competitor. Rather their focus would be on what it says about your desires and the sexual connection they thought they had with you. The fbb would just be the agent of this revelation.
Just imagine the scene if your girlfriend/wife walked in on you, and the difference in her reaction to whether you were shagging a slim pert blonde, or if it looked at first glance that you were getting humped by a bulging muscle man.
There would be no way back in the former situation, but in the latter case, once your partner realised (if she hung around long enough!) that the owner of those huge thighs, dense glutes, broad muscular back and shoulders and big strong arms was actually a girl, she would probably be more shocked and confused than angry and might even try to understand how you two could get past this.
Some relationships are more mature and broadminded than others about accepting that you both have kinks and desires that don't completely correspond with what the other can offer, and while few women would accept an open relationship, they might try to channel those kinks into your own love life if the rest of the relationship was healthy.
Unless your wife/girlfriend has any dominant tendencies however, seeing her man emasculated by a woman who effectively looked more masculine than him would also be a hard thing to get past, in terms of how she regarded her man.