There are 5 confirmed romance options in the Witcher 2, though the term romance is used loosely since most of the encounters are meaningless sex. Triss and Ves are the only romanceable characters in the sense that there is an actual depth and consequence to the relationship.
In all Geralt's liasons 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 Act 3 if he follows Iorveth's path and completes the quest The Secrets of Loc Muinne.
It should be noted 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.
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 Act III when she is captured. Regarding sexual encounters, 2 have been confirmed.
The opening scene of Geralt's story is him getting up as Triss lies naked beside him as someone enters, so there isn't a romantic encounter as such, but is indicative of one.
Act I Edit
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.
Mottle is one of the elven women that can be saved in Flotsam during Chapter I if Geralt chooses to follow Iorveth to Vergen. If Geralt saves her, she will want to reward him in Chapter II with "joy". She also gives Geralt the Key to the Tunnel of the Founders after receiving his "reward".
She can be found just outside the town of Vergen 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.
Ves is the first woman to appear in the story, and is the only other character aside from Triss who is actually romanceable in Act 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.
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 to try the device, then meet her back at the inn