On the one hand, Infidelity is infidelity.
Regardless of one's orientation.
But In truth, men do not come in two styles. Straight and Gay... 10% of men prefer only other men... 10% of men prefer only women.
The rest have some admixture of interest in both, from nearly none to quite a lot... and those sexual drives are difficult to deny, 100% of the time, every day of our lives.
That means that the majority of men have some level of fascination with other men's cocks... its why porn ain't hard unless there is a rigid cock involved.
I can resist temptation 1,000 times, and then fail once... am I a douchebag?
Or am I one tenth of one percent douchebag?
Or could there be other factors involved... could I have a wife who has lost interest in me sexually, or make me feel like she is doing me a favor to allow me to touch her?
Might I be going thru some other crisis, that makes me feel like none of my needs are ever met? Do I faithfully provide my labor and my income to the support of my wife and family, only to be denied the physical comforts promised when I entered into marriage?
There are lots of reasons for infidelity, not all of which are indefensible. And there is no such thing as a homewrecker... a solid relationship can not be wrecked... either the cheater is amoral... or the spouse is not meeting their mate's needs in some regard.
Because of what I have to see as the Normal amount of bisexuality in most males, and the fact that men Can separate the pure sexual release from involvement with their heart... it is entirely possible that some men come to the conclusion that having sex with another man is LESS of an infidelity, Because it does not involve their heart...
(they may like sex with men, but do not feel 'love' for them)
I can not put myself in the position of judging anyone... The body yearns, and the heart has its sorrows and its needs...
But I would say that the only ethical approach is that your mate should know of and agree to any activity outside the marriage bed...
Or you should not be married.
Sex with other people exposes your mate to risk.
They have the right to assess and decide if that is a risk they are willing to accept.