After taking Siobhan's place, Bridget tries to repair Siobhan's relationship with her husband Andrew. Though Siobhan and Andrew were once passionate, she grew out of love with him and instead fell in love with her best friend Gemma's husband Henry, with whom she started having an affair. Feeling sorry for Andrew and not wanting to disturb "her" friendship with Gemma, Bridget tries to play a caring wife to Andrew and firmly rejects Henry.
Gemma finds out about the affair in "It's Gonna Kill Me, But I'll Do It", and in the next episode "A Whole New Kind of Bitch", she tries to coerce Bridget into sleeping with Henry so that Gemma could get proof he's cheating on her for the divorce. Bridget spends the episode trying to figure out whether or not she should do it even if she'll be exposed as Bridget and not Siobhan if she doesn't go through with it. Her moral qualms seem to be that it would be wrong to cheat on Andrew and to have sex with Henry under the pretense of wanting to have a romantic relationship with him. Though the first is based on the questionable premise of her having a real relationship with Andrew, neither are bad reasons to feel the act would be wrong; however, it is problematic that the issue of the act in question being rape is never presented.
Rape is the act of sex performed without the informed consent of the participants, and due to the masquerade in place, Henry is unable to truly consent to having sex with Bridget. He had a consensual sexual relationship with Siobhan, but she's out of the picture now. If Bridget were to pursue a sexual relationship with him and not announce her identity, she would be raping him because he would only consent to the figure of Siobhan. He doesn't know who the hell Bridget is, and Bridget is well aware he doesn't.
This reminds me of the similar plotline in the Buffy the Vampire Slayer episode "Who Are You?", in which Faith swaps bodies with Buffy and takes over her life. Partially out of malice towards Buffy and partially out of her own lust, Faith has sex with Buffy's boyfriend Riley while disguised as Buffy. Riley believes he's having sex with the woman he loves, and the tone of the scene is about Faith feeling touched at the idea of someone loving her. After the body-swapping is sorted out, Buffy feels hurt when she finds out Riley cheated on her accidentally.
Buffy: "You slept with her."In the following episode, "Superstar", Buffy rages at Faith to MarySue!Jonathan. Jonathan insightfully describes how it's not really about Faith; she wanted Riley to be able to tell Faith wasn't her and now feels insecure. He advises her to forgive him, but Buffy worries that maybe Riley now likes Faith's sex better. Jonathan then advises Riley to tell her that's not true. They take his advice and things get better between them.
Riley: "I slept with you."
It is never even implied that Faith has victimized Riley by having sex with him without him knowing it was her. The only reason shown for Buffy to be angry at Faith is for her having sex with Riley as an insult to Buffy. Faith violated Buffy's monogamous relationship with Riley and as a result made Buffy suspicious of Riley's loyalty to Buffy. It's played like Riley cheated on Buffy with Faith, except for the body-swapping plot making it so they can have the drama without blame being cast on Riley to ruin the relationship. In truth, Faith raped Riley by having sex with him without first obtaining his consent, and Buffy and Riley should be mad at her for that primarily instead of Faith's disruption of their monogamous relationship being the only thing that bothers them.
This act of rape via disguise is commonly ignored in popular culture and in law (this article describes an attempt to change that). This trope (TV Tropes calls it Bed Trick) has been performed with men raping women in various movies such as Revenge of the Nerds without being portrayed as rape, but as time goes on, I think people are more able to recognize that as fraked up. It is particularly insideous when used with women raping men because people don't see men as sexually vulnerable the way women are, nor are women perceived as being capable of being threats. This is also fraked up, and people--especially feminists--need to realize this.