Because consent ISN'T sexy.
Though the organization is very well planned out, and it truly is well-meaning in it's goals, the message is counterproductive, even self-contradicting.
Firstly, if consent is sexy, then not giving consent is unsexy. This can easily pressure people to say yes in order to be "sexy". As we have learned, being pressured to say yes is actually sexual assault. While not everybody may automatically assume that not giving consent is not sexy, it's very easy to subconsciously think that when reading the slogan.
It also diminishes the meaning of consent. Asking for consent shouldn't just be because it's sexy and it turns people on. It is a mandatory right that every person should have. We should ask for consent to respect our partners, and to prevent sexual abuse or assaults.
Also, if consent is sexy, then sex without consent is unsexy. However, this is not true. Sex without consent is rape, plain and simple. It weakens the actual act of rape, something that should be considereed completely intolerable.
Lastly, some rapists don't care if consent is sexy or not. They feel like they are entitled to sex, and will do anything they can to get it. The problem isn't that they are unsexy because they did not get consent, it's that they are rapists because they did not get consent. We must educate people that they are not entitled to any person for sexual desire, as this is a form of sexual objectification.
Describing consent as sexy, or even anything, will lead to problems. Consent is a mandatory human right that everyone should be able to give or not give.