Hello Dave,
The Mark of the Beast is the anti-Shema. Shema is basically the prayer "I love God with all my heart, soul, and mind." Deuteronomy discusses how it should be placed on the forehead and hand. The anti-Shema is basically saying that Nero or Domitian was requiring that everyone publicly required to proclaim that they were gods. Basically, one way to root out Christians was to force Christians to perform Cesar worship.
Now, the mark of the beast is not necessarily the gay marriage thing in my opinion, although I do wonder why we need the government to be involved in marriages in the first place. Remember that Jesus did meet the Roman soldiers who asked what they should do. Jesus didn't say they should disobey their commanders when things got dicey. And I'm sure Romans soldiers were told to do a lot of things which were far from "good". That being said, I would try to avoid jobs which caused me discomfort like serving wedding cakes at a homosexual wedding.
I also have a problem with the Pledge of Allegiance because my allegiance is not really to the U.S., it's to Jesus. But I still do it because it is part of my job. They aren't making me say that the U.S. should be worshipped. The "under God" clause makes the entire thing far more palatable.
For my opinion, it's clear that the number 666 was clearly referring to Nero or Domitian (Nero's "reincarnation"). This was a circular letter to 7 churches. As such, it may have been intercepted. John used imagery quite a bit to hide certain things which could be used against the Church by the Roman authorities. Calling the leader of the Roman Empire a man doing evil things who was only the puppet of the Devil would definitely be used against Christians. 666 is the transliteration in Hebrew. Greek writers changed it to 616, which is the transliteration for Greek.
An antichrist is anyone who denies the Jesus Christ, either his divinity, humanity, or the other aspects of Jesus. They oppose Christ. John mentions this both in First and Second John.
For example, George W. Bush might be considered an antichrist for saying that Christians and Muslims worship the same god. Of course, he's likely just ignorant of what Muslims believe. So, I won't judge him. I only remember him saying it once.
The Antichrist is obviously Satan.
But all recent Presidents and most politicians have denied that Jesus is the true Messiah. Most like to say that there are many ways to heaven and no ones knows which religion is right one. They deny Jesus. John would call these people antichrists. Now antichrists can still repent and become Christians. The most famous of these was Saul/Paul.
If the Mark is coming back soon, it will probably be to requirement that Christians can't say Jesus is the true Messiah and the other paths lead to Hell. You already see some of these restrictions on the proclamation of Jesus in England, Canada, and Australia.
I don't claim to be perfect and I don't know if I'm inspired by God. So, If you think I made a mistake please point it out. I'd love to learn more.
Marty