"I was reading up on your 2013 post about Anthesteria in preparation for tomorrow night when I came to the bit about covering your altars to prevent miasma. My main altar is in my bedroom. Should I be covering it every time my husband and I have sex? Have I been polluting my altar for years?"
The post in question can be found here. In it, I talk about covering your main household shrine as a sort of tribute to the fact that for the final two days of the Anthesteria, all temples were closed besides the one of Dionysos. Seeing as this festival is partially an underworld festival, it carries miasma. It is a practice I have adopted for anything underworld-related; I also do it when someone close to me dies, for example, until I can purify myself.
Sex in Hellenismos is.... complicated. Hellenic society was complicated when it came to sex; the ancients saw sex as completely natural and--unlike many today--had no inhibitions and very few taboos when it came to straight up heterosexual sex. Anything else had societal stigma's attached to it. They did however have many thoughts and rules surrounding ritual cleanliness--miasma and katharmos.
The ancient Hellenes viewed sex differently than we do today. In general, they accepted lust for sex as a disease--as madness that needed to be given in to on occasion to preserve sanity. It was the result of a disturbance of the healthy equilibrium between body and mind. Sexual desire made one loose mastery of their intellect--which was a huge ideal in ancient time. Women shouldn't be encouraged to give into that madness too often, but husbands did have the legal obligation to relieve this need in their wives at regular intervals so as to prevent them from becoming dangerous.
Mikalson in 'Ancient Greek Religion' mentions that intercourse led to miasma and that a bath was required before entering a temple after intercourse as a form of katharmos. He, however, does not give a source, and I don't know one either. It is a reoccurring idea, though, mostly centred on the male's excretions during the activity. The Hellenic religious organization 'Labrys' echoes the sentiment, but also without sourcing.
I keep my altar in my bedroom as well--as we live in a one bedroom apartment and we have sliding doors to corner off the bedroom during the night. During the day the space is part of our living room. What I do have is curtains to shut off the bed from the room beyond and thus creating a temporary barrier between the bed and the altar. When I make love to my girlfriend, I'm mindful to keep these curtains closed. Honestly, I mostly do it because otherwise I feel like there are Gods watching with popcorn, but hey, miasma is an issue too.
We all incur miasma, every single day of our lives. It has nothing to do with sin, shame or guilt. Miasma is a consequence of living. We breath, make decisions, come in contact with others, and along the way, we become too human--for lack of a better term--to petition the Gods. The divide between the purity and cleanliness of the Theoi and our human mortality and imperfection, keeps us away from Them. Miasma is not about being physically dirty, although that is a part of it, and katharmos is not about becoming physically clean, although that is a part of it as well.
After a lot of research into the workings of miasma, I have come to the conclusion that miasma is linked to distraction. Anything that takes your mind off of the Gods during ritual can be considered miasmic. For example, murder causes miasma (when not committed as part of a war, soldiers were not tainted with miasma for killing their enemies), but only once other people became aware of the fact that you had committed an act of murder. As such, if you were exiled and you travelled to another town where no one knew what you had done, in essence, you were not miamic to the rites and people around you. If you can keep your head in the game the morning after and you have taken the proper steps to clean both yourself and the space, then by all means, do the rites. If you can't, well, then it doesn't matter where the shrine is located, now does it? If you mind is still on last night's events, you have no business petitioning the Gods anyway.
Real talk: we don't live in ancient Hellas anymore and while I am a huge stickler for practicing Traditional Hellenismos, not all of us have a huge altar in our garden that we can perform sacrifices at twice a day. Most of us don't have a wood stove or some such to offer at. We live in the now, and as such, we are forced to take certain liberties and deal with the consequences. As such, I would encourage a barrier, but if you don't have one and you have sex next to your household altar, make sure you and the space are clean when you give sacrifice. Sprinkle khernips, take a bath, change the sheets, make sure your mind is entirely on the ritual and not last night's marathon session. Give the Theoi their due in an area that is clean and tidy. Compartmentalize.
So no, I don't think you need to cover your altar whenever you have sex (although I would encourage some sort of barrier between your altar and the bed out of respect). I do think you need to do much more extensive cleaning of the space if you have your altar in your bedroom and you've just had sex. The sight of crumpled sheets and your sleeping lover would undoubtedly bring distraction, and that I would warn against. As a final note: enjoy each other. Share love. These things are far too important to hold off on. Your worship matters but it can be adapted to suite the needs of the entire household--and at its core, that is the main focus of Hellenismos: providing a healthy relationships with the Gods to our family so they in turn may bless us and guard our household.