Personally, I think it's a little uncouth to leave your hosts to clean up your funky sheets and throw out your condoms.

If they're relatively newlyweds, it'd be kinda silly to expect them to not have sex. Any guest of mine is allowed private time, and to please remember to change the sheets if necessary.

Yes, I'd advice not to do that, no matter how much privacy they're given. I'd say its a basic mannerism. I don't know how to explain and justify it though.

The answer to this complicated question lies in exactly whom I have allowed to stay in my home and why. If they are friends whom I allowed to spend the night after a party, why would I expect them to abstain from something that I probably would not? If they are business acquaintances to whom I have extended my hospitality for business reasons, then I would certainly abstain and would expect them to behave likewise. If they are family members, such as juvenile nieces or nephews, I would assume that they would have enough respect to consider their surroundings and behave accordingly or if they are aunts & uncles, grandparents or in-Laws, then I would abstain and would expect them to do the same in a sign of mutual respect. If they are an unrelated someone that I have allowed to stay in my home by a friend's request or out of necessity or because of some particular emergency (for example: a fire in their own home, a death in the family, etc...) I would abstain and would also expect them to do the same. There might still be other circumstances that I have not previously mentioned where I would see such behavior as acceptable or understandable, but at the moment, I cannot think of another.

There are a number of factors that should be considered and I have seen several of them mentioned already. The biggest I think is what type relationship do you have with the owners of the home. There are some who would probably expect you to have sex just as they may be doing as well. In some instances the host and hostess may join in and you both with them. This is not unheard of in open relationships. There are times when any sexual activity could be very out of place. So as I say, you have to know the couple you are staying with and what their standards are. Today many are open to sexual activities and there are also those whom would frown greatly.