IIRC the general idea is that the Stargates aren't just opening wormholes, they're deconstructing matter as it crosses the event horizon, then transmitting that matter through the wormhole for the gate on the other end to seamlessly reassemble. Presumably wormhole travel isn't survivable if you just sent a live subject through...or the throat is so small that nothing larger than an atom or a radiowave would actually fit anyway. Note that there is also a transit time that the traveller cannot perceive, so while it feels subjectively instantaneous, objectively they loose a few seconds with each trip.Ok, I hope they explain that in future episodes. I'm all for the series rule book, but watching the episode I kept thinking, these are portals why not use it as such. I'll accept that you can only go one way if they are consistent with it.
Whatever the made-up explanation though, the main take-away is that these are devices with very carefully implemented restrictions, mostly for safety reasons. I mean imagine what would happen if two-way travel were a thing and you dialled a gate that was submerged under an ocean of water, in a vacuum, a poisonous atmosphere, or even with a slightly different pressure. And that's not even including things like radiation or invasive fauna (anyone seen 'The Mist'?) It could cause all sorts of problems at either end.