If you think about it, "because it's sexy" basically translates into "hurr durr...because I want you to." Which is a non-answer, if you ask me - of course you want me to do the thing you're telling me I should do, but what's the justification? Where's the convincing argument?
("Do x because it's sexy" really usually means "Do x because [I think] it's sexy," which follows naturally do "Do x because I like it/want you to.")
Now, for some people and in some topics, "because I want you to" is enough justification. For instance, if someone asked me to wear lingerie for them, or basically anything else rather low-level and usually not too much effort, time or cost that is placed in an arena where there is a general willingness or desire to please the partner. Sex is a great example because you should want to please your partner during sex. Business/monetary transactions are a great example of where this should never fly. The management of how your body appears to the world is another one. Your body's about the most personal thing you'll ever have, inhabit, own, (whatever word you use for it) and it controls you while you control it.
While "to look sexier for my partner' might be part of a list of reasons a person might want to lose weight, become more muscular, gain weight, hell cut their hair or something, I agree it shouldn't be the only reason and I certainly think any person telling me I should alter my body simply because their desire is sooooo extraordinary as to warrant it is not only justification but a gross demonstration of how that person thinks about his or her right to tell you what to do in life. And then after considering their lack of boundaries and desire for control/influence over you, then you should move on to the fact that they think "because i want you to" is good enough to convince you.
My hackles go up with a bang, immediately, when anyone comments on what I should do with my body or whether what I'm wearing is appropriate/'not attractive'. It's a sensitive topic but moreover it's my body and my choice. (This comes up a lot because I have a number of tattoos and plans for more. I love them and think they add to me; I certainly consider them embellishments, ornaments, attractive additions. However, I have family members that think it is appropriate, even necessary, to tell me that the tattoos make me "ugly." It's my god damn body. Stop thinking you have the right to tell me what to do with it.)