The Witcher 2 romance

there are 5 confirmed romance options, though the term romance is used loosely since most of the encounters are meaningless sex. Triss Merigold and Ves are the only characters to whom a term romance would apply correctly, in the sense that there is an actual depth and consequence to those relationships.

In total, Geralt's liaisons can include Triss, Ves, the Succubus, Mottle, and prostitutes at either of the two brothels. He can also have a sex scene with Cynthia in Chapter 3 if he follows Iorveth's path and completes the quest The Secrets of Loc Muinne. This however is only possible if Geralt chooses "I want to try the device." at the end of the quest.

Triss Merigold
The opening scene of Geralt's story is him getting up as Triss lies naked beside him in a tent as King Foltest's messenger enters, so it isn't a romantic encounter as such, but is indicative of one happening prior to that. It is a typical relationship's "morning after" scene.

Triss Merigold
Triss is Geralt's default love interest, and the game begins with her and Geralt being in a relationship. It is easy to build your relationship with her by talking to her at each location where the option is available. You can also choose to save her in Chapter III when she is captured. Regarding sexual encounters, two have been confirmed. This opportunity arises in Act I in the quest The Rose of Remembrance. When given the option to go alone or take Triss, choose to take her with. After a battle partway through the quest, the ground will collapse beneath you, sending you and Triss plummeting into an ancient elven bathhouse. After failed attempts at escape it becomes apparent that you will be spending some time here. Talking to Triss and suggesting that she take a bath will initiate a sex scene.

At a Crossroads: Scoia'tael: The Succubus
The Succubus is one of two possible parties to side with during With Flickering Heart quest, yet available only if Geralt chose to follow Iorveth at the end of Chapter I. The other party being Ele'yas, the Succubus' envious lover. At the end of the quest the Witcher will have to choose either to side with the elven lover or the temptress proving her innocence instead of slaying her. Apart from providing Geralt with few "material" rewards the Succubus gives our hero something more... some quality time with her, as she describes it "unearthly pleasures".

At a Crossroads: Scoia'tael: Mottle
Mottle is one of the elven women that can be saved in Flotsam during Chapter I if Geralt chooses to follow Iorveth to Vergen and if he chooses the rescue over pursuing Loredo (the very last choice in Chapter I), she will want to reward him in Chapter II with "joy" considering it a small compensation compared to Geralt's heroic deed. She also gives Geralt the Key to the Tunnel of the Founders after receiving his "token of gratitude".

She can be found just outside the town of Vergen, past the main gate from the market to the left where she sells herbs and crafting components. Of course, if Geralt does not save the women from the tower or sides with Vernon Roche, she is nowhere to be found.

Note that in some versions (AU, possibly others) of the game the reward is unavailable as Geralt declines automatically.

At a Crossroads: Vernon Roche: Ves
Ves is the first woman to appear in the story, and is the only other character aside from Triss who is actually romanceable in Chapter II, Roche's path.

Romancing Ves is possible after completing the Ave Henselt! quest, where you are challenged by Ves. After she yields, you can go and visit her in the Blue Stripes camp outside the Kaedweni Fortress. Talking to her triggers the sex scene.

At a Crossroads: Scoia'tael: Cynthia
Cynthia is involved in the quest The Secrets of Loc Muinne, a new quest added in the Enhanced Edition and only available on Iorveth's Path. Upon completing the quest, Cynthia can become a sexual partner for Geralt. Choose: "I want to try the device." at the end of the quest, then meet her back at the Eternal Fire inn.

Trivia

 * In the Australian version of The Witcher 2, the flings with the Succubus and Mottle are edited out of the game. Geralt will automatically decline the invitations because of this censorship. The programming for the encounters is still in the game's code and can be restored with a fan made patch.
 * Most of the sex scenes are generic and have few (three?) variations that use exactly the same script but change most likely only the head/face textures of a woman. Example: Mottle scene would be identical in terms of movement pattern with some prostitutes'; Cynthia's scene would be different than Mottle's but same as some other prostitutes'. Basically it's "take a scene A and swap face, take a scene B..." etc.
 * The only unique scenes are: Triss' and Ves'.