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Saying yes to Puck doesn't necessarily mean she's selling her morals for a boyfriend, Rachel reasons. It's important to have a healthy, active love life; it lets people know that 1) she's not neglecting herself to further her career, 2) she's an acceptable object of desire they can long for and lust after from a distance, and 3) show choir is her passion even if she has a boyfriend. How are people supposed to know she's in this for good if she doesn't have any potential distractions to relegate to the lower levels of her priority list?
Also, it lets little creeps like Jacob know she's off limits in every possible way, which is never a bad thing.
And Puck, well. He's bullied her for so long she's given up on keeping track of his offenses, so dating him shows she's capable of forgiveness. And while people are busy giving him slushie facials for dating her, they're not messing up her hair and clothes.
Besides, she and Puck do look good together, and pursuing a career in show business means sometimes having to date people you don't really like so you can wear them as accessories to premieres and parties. Puck is pretty good arm-candy: his build is great, and he's confident enough to make an impression but not determined enough to steal her thunder. Walking down the hall with their arms linked is good practice for future red carpet walk-downs.
So it doesn't reflect on her person if she decides not to say no. At least she's not letting him get to second base, and anyway, he also wants someone he can't have right now, so it's not cruel of her to pretend she likes him more than she really does, because he's pretending to like her more than he actually does as well.
And if she turns out to enjoy those pretenses more than she expected to, well, that's only because theater is her calling, and she enjoys acting in any and every circumstance.