Is it possible to be a good Marxist while being opposed to gay marriage?

There would be no reason for secular people to get married under Communism.

And how can you call yourself a Christian?

If you know these people are going to hell any way then why try to stop it? Just seems like you want to put your noes and other people's business.

Just abolish marriage altogether

it's a good start.
next you should start being opposed to all marriage

Because marriage is a religious thing.
There should be a non religious equivalent for whoever wants it.

Better question should be, can you support marriage(an bourgeois institution) as Marxist?

Yes by being opposed to the institution of marriage, which everyone here should be. That you're a spooked fuck is more indicative of a need to read Stirner.

posting as a christian communist should be a bannable offense

Marriage is a civil thing, it was not always tied to Christianity. There's no reason why in modern times it can't be open to gay people. A gay marriage existing does not ruin your own, if the catholic church wishes to not marry any gay couples that's their business but they can't prevent any other church or legal authority from making the marriage a reality.