That is not relativism. Two people, each with the “facts on the ground” in front of them, weigh these facts, and then make a decision. Neither one is necessarily “right” or “wrong”. Even moral theologians come to different conclusions — usually with some nuance of thought, so that it is not as simple as saying “you can do X, always, no matter what” or “you cannot do X, never, no matter what”.
It is not intrinsically evil to be physically present in a place where a sacrilege or an abomination is taking place. Otherwise, for instance, if someone at the ceremony fell dead of a heart attack, the EMTs could not come and get that person without sinning, for “coming and getting that person” would require their physical presence. Of course they can. It would actually be evil for them not to come, to say “we’re not going into that place and rendering aid”. First responders went to the Pulse nightclub in Orlando and rendered aid, as they should have done. No doubt some of these first responders were faithful orthodox Christians of various denominations. Chick-fil-a even prepared and donated food for the responders, mirabile dictu, on Sunday! (Note to readers outside the US: Chick-fil-a is owned by evangelical Christians and they close on Sundays. They are a wildly popular chicken fast-food restaurant chain.)
Okay, move forward from there. Abel is “marrying” Baker. Abel’s father is invited. He tells Abel “I cannot do that”. Abel says “okay, but if you don’t come, you’ll never see your grandchildren again”. (Abel has children from a previous heterosexual relationship.) Abel’s father says “well, that’s pretty dirty, but if that’s the situation, then may I come and sit on the back row?”. Abel says “that’s not being very supportive of Baker and me, but at least you’ll be there”. Abel’s father says “and will I then get to see my grandbabies?”. Abel says “sure”. Has Abel’s father sinned? I don’t think so.