It's a phrase one says when they aren't really kidding but instead use this to take off some of the "truth hurts" pain. While people are laughing because you said sike, you throw in..."But forreal though.."
Jim: Dude your girlfriend is pretty ugly
Mike: What ?
Jim: Sike, naw...but forreal though...she's pretty ugly
While our lifespan is the number of years we will live, our healthspan is the number of healthy years we will have and our sexspan is is the number of years we will continue to be healthy enough to continue having an active sex life.
A lighthearted reaction upon receiving disappointing news of some sort. Usually something that isn't a particularly a big deal, esp. when it primarily affects someone else - sort of a mild schadenfreude. Sometimes doubled or tripled, and possibly said in a sing-songy voice for added comedic effect to imitate a muted trumpet, which is how the meme originated.
You didn't get any napkins with your takeout pork fried rice? Womp womp.