physical~ gay sex.
They don't have the body parts to complete the task at hand!
mental~ Thier pure love for each other. not becuase of gender (like you said) but becasue they like each other's mind and they like to talk and be with them forever.
Male to Male or Female to Female - These Type of Friends already do that!
I have several Friends like this and so does my wife!
spiritual~ With Gods help thier marriage will last and they will receive His love.
Only when they truely seek His WILLl minus what they want or think it should be!