<human>: Could you distill the essence of the text into 1-3 sentences?

Relevant info: I've been with my SO for 5 years, married for 2. He (we'll call him Gus) has been with his SO for several years, married for 3 with a kid. We live on different continents.

Background: I met Gus through mutual friends when I was 17 and he was 26. I didn't have many friends at the time so we hung out every once in a while. Come to find out Gus asked my dad if we could be in a relationship. My dad, concerned, asks me if I'm okay with this, I tell him in no uncertain terms, "NO. I thought we were just friends." So that gets shut down. We go our separate ways, still friendly but we don't do things together anymore.

Cut to 8 years later. I'm in the states, Gus lives in AUS. We're both married, him to a girl he started dating almost immediately after our awkwardness, me to a guy I met at college. Gus and I are facebook friends, so occasionally we come in contact on each others' statuses and links, but we're not really involved in each others' lives. I posted something yesterday, he commented, I replied, just a simple pointless internet conversation.

I wake up this morning, I have a facebook private message from Gus with the line, "Just wanted to let you know, I've always loved you."

So. What do I do now? Should I reply? What do I say? Should I try to play it off, or just shut him down immediately?  This is a guy who is eight years older than me, married with a kid, and I am married as well...I don't want to lead him on AT ALL if this is for real, but I don't want to hurt his feelings if this is some kind of (poorly conceived) joke. I could definitely just ignore the whole thing (I haven't actually clicked the message, so it hasn't registered as read yet) but again, if this is just some kind of joke... I don't want him to think I'm weird for overreacting.
<bot>: A married friend may have just confessed love. Do I try to dissuade him, laugh it off, or just ignore it?