This is as much a question about general social dynamics as it is about house parties in the LS.
When there is a 2+2 situation, the dynamic is pretty clear. It's for precisely this reason that many couple..we included...choose to meet new couples for the first time in a stress-free zone, such as a a restaurant for dinner, with no expectations. People can decide about chemistry, trust and so on.
When there is a large party, say 10+ couples, the combinations are high and moreover there is always someone/some couple taking a breather. As you point out, getting sore, having to recharge, or simply reconnecting with your partner and so on is part of biology. Not everyone wants to play all the time. We first become exposed to these social dynamics in grade school. Some people always want to be in the middle of things, others want to spend their time on the periphery diving in only when the mood suits. If the number is large enough, things work out pretty well.
The most challenging situation is when there are more than 2, but no more than 4 or 5 "units"--here, a unit is a couple. If a couple (or part of a couple) wants to go off and do something else ---whether vanilla or LS situations--it feels a bit awkward. If the existing group is really experienced, and they are inviting you into their inner circle, then they know what it was like when they joined, and they'll make you feel at ease. The problem arises when there are real differences in expectations. If you say for the first time, we'll absorb the atmosphere but play only with each other...this was our approach, BTW...and they know this in advance and are okay with it, then the pressure around expectations is reduced.
As to germs, that's an LS risk. Just kissing other folks will lead to typical upper respiratory infections with at least some frequency. Nature's way of telling us to take it easy. You have to decide your risk tolerance. Again, just playing with your spouse the first time will limit the type of risk you take and also enable you to get a sense of how the others manage the germ issues.