It certainly gave the protest more impact, having it at a funeral...
You have to understand how powerful religion is. You weren't brought up in an overly religious family, but if you are, it's burnt in your mind the same way morals and values are. Just like you oppose murder, they oppose homosexuality.
They protested at multiple funerals of dead soldiers because they believe that the US Army is a symbol of America's tolerance for homosexuals. You know, the whole "Don't ask, don't tell." policy. They feel that being homosexual is just wrong. Just like being a rapist or a paedophile is wrong. I mean, the paedophile can't help what he loves right?
They think that the failing Iraq war and the dead soldiers are a sign from God that it is time to end homosexuality. Just like the Flood was God's sign that the world had gone wrong.
Religion is powerful enough to cause people to blow themselves and tons of others into ashes (Terrorism). It's powerful enough to torture and kill thousands (Spanish Inquisition, Holocaust). It's powerful enough to start wars (Crusades, French War of Religion, the Reconquista, etc.) and powerful enough to declare emperors (Pope Leo III) and start coutries and send people off into the unknown to spread it around(Proselytism).
It's certainly big enough to send a couple of people to stand in front of a funeral with signs to protest something that is very wrong to them.
I'm not supporting their beliefs or position on homosexuality. I'm just saying that they've got their reasons. And yes, this is America, and no, there isn't a set law banning homosexuality, but there's still tons of discrimination against gays.