Compare how they react to one another. In Josie, Alexandra clearly wants Alan, but she's a pest. She and Josie are often at odds, and Alexandra is probably there only because she bullied her brother. If Skip and Augie were romantic rivals, why would they both be going everywhere together willingly? In the Headless Horseman episode, Skip and Augie both gravitate to a new girl together, which bothers April. If they were rivals, you would think one would kick the other out. And likewise when April goes with another guy in the race as navigator, Skip and Augie barely fight as they have to get their girl back. In the boat, April's response to them fighting is, "Will you two decide which one of you is going to serenade me?" She's completely fine with the slight competition for attention but flirts back and is no doubt getting it from both of them.
Now this raises the question, what were they really going to do when they went inside that spooky old house? The song does say they were just hoping to get warm; sex totally would get them warm. There's more evidence than there is length to April's skirt.
- While the idea they are in a poly is very possible April does mention having a western outfit back at home and Augie does have a garage with his name on it.