Pairs was a naughty adult arcade game released by Strata and Incredible Technologies back in 1994. Yes there really are "adult" arcade games, actually there are lots of them. Most people just don't run into them very often because they kind of have a rather limited market (except of course in Japan, where there are dirty mahjongg games all over the place).
Do you remember the game "Memory"? You know the one where you take a deck of cards and lay them all down in a large rectangle and try and uncover matches. Well this is memory, but there is a big picture of a naked (or semi-nude) woman underneath the cards. The picture starts out as being kind of grayed out, but it comes into full focus and color after you remove the last pair of cards.
The game is played with a standard deck of cards (52 cards plus two jokers). These are laid out in a 6x10 grid. You begin by selecting a card, it turns over, and then you try to select another card that matches. If both of your cards match, then they are both taken away (and a bit of the picture is revealed underneath them). If they don't match, then they are flipped back over and you must try again. The number of chances you have is kept track of with hearts at the bottom of the screen, you get an extra heart each time you guess correctly, and you lose a heart each time you guess incorrectly.
The game has eight different women to choose from, and there are four stages for each woman (the pictures get naughtier at each stage). Once all 32 of those stages are completed, you get to play several bonus stages that have pictures of several women together. I haven'y gotten that far, so I can't tell you exactly how naughty the pictures get.
This is a really fun game, despite its obvious adult trappings. I have spent quite a bit of time playing this one, as it makes you think (and gets quite mean about mixing up the cards later on). This isn't the best place to look if you want porn (unless you enjoy really skanky looking women who were obvious Penthouse rejects, in that case this is the best place to look).
This game was released as a conversion kit, and can be found in a wide variety of different arcade cabinets. It will most often be found in bartop or converted cocktail units, due to the fact that this game was designed for bars, and bars prefer those form factors.
The best place to play this game is by using the MAME emulator. Actually that is the only place to play this game, unless you buy an actual machine. Now as far as buying a real machine goes, I would advise against it. It is hard enough to explain to some people why you have full sized arcade games in your house, and trying to explain full sized pornographic arcade games is just more trouble than it is worth.