In which of the houses do I have to look for a love affair of somebody married? 5th or 8th? Or other?
It depends very much on what branch of Astrology you are using here. If you are looking at natal charts, then I think there's a real problem. Nativities only show potential and even then that potential isn't realised all in one particular moment. So I'd say you're looking more at planetary relationships and then using derived charts such as solar returns, or looking at directions (primary, secondary, solar arc, etc), or transits, for times when those relationships might be activated.
Traditionally Venus is associated with adultery, especially in configuration with Mars and or Saturn, so your starting point would be to look for any configuration involving Venus and Mars and or Saturn. Some modern Astrologers take Uranus in the seventh as an indicator of divorce (and adultery is one of the main reasons leading to divorce) so you could look for that. However, I'd stress that the configuration is not proof that adultery will take place, you would need to look for those activations as well, which would raise the chances.
If you were looking at adultery through a hororary question, then I'd use the seventh, house for a partner in a longish term relationship or a question where adultery was known (we had such a question recently in the horary thread). For one night stands or suspicion, I'd use the ruler of the fifth as the significator of the third party. Like Ronia, I'd also take a look at any connection to the twelfth, as it's concerned with secrets and the hidden (adultery is rarely open), and in the question I referred to the 'adulterer's significator was actually in the twelfth house. The eighth is also a possibility, not because of the modern equation with sexual relationships (which is more fifth house) but because like the twelfth, it's inconjunct the Ascendant and therefore 'out of sight'.
I saw one reference to Mars and Venus setting into the Sun - that is withing about 15 degrees of it. Planets near the Sun cannot be seen, so the two principle general significators of possible adultery, being 'out of sight' might well point to something going on.
Edited to add:
There is a Lot of Adultery which is defined as the degree exactly opposite the Lot of Marriage (ASC+Venus-Jupiter). So that too might feature in your analysis, especially in a horary chart. For nativities, you could look at directions and transits to this point