The Multiplayer mindset my friends and I have.

I've got a group of friends playing Civ with me since the game since Gods & Kings' release. I really enjoy playing with them, but there's one issue overall. It turns out we're all Builders. We don't focus on military at all. We leave each others alone, and we feel bad for attacking anyone. If there's occasions to win, we'll often delay the victory just to enjoy the end of the tech tree. Now this is fun for us, but y'know, I feel like we're not enjoying the game as it should be. We're not skirmishing each others, we don't capture PC cities...

I'd like to change that a bit, spice things up, open their horizons, maybe, but not too brutally. We usually play Standard maps on continents, half Human players on Prince, half AIs. Anyone could give me tips on how to improve our experience?

Personally, I prefer to win the game. Contrary to your beliefs you are not really killing your friends, its just a game. Woop some ass, they'll get the idea. Beeline to pikeman, go pillage and they won't be able to keep up building wise. Just screw with them till Artillery then run them over.

Try and play with some different people. Or invite someone else into the mix who knows how to conquer

I had the same problem with my friends. After the first multiplayer game we played they stopped playing with me for a while because they thought that "military victory is cheap".
One of them happened to be my closest neighbor (was a huge map with us 4 and a truckload of AIs), and I sent a small army of swordmen and sacked his capital, and they all ragequit.

After aboot a week or so they were over it and we started playing together again, and they learned to deal with my playstyle and started actually building defenses and some military units early. Had a lot of fun playing together since then.

My advice to you is to just pick a good warmonger civ next game and go on a rampage so they'll learn to deal with war, which is an important aspect of civilization (if not waging war, then at least defending yourself when war finds you).

"If you want peace, you must prepare for war. If you want war, you must also prepare for war. The lesson is: always prepare for war!" -Mogul Khan