It cheapens the seriousness of the decision.  I'm pro choice but would be horribly put off by seeing anyone wear it.  And you know high school girls would be wearing it just to look like "bad girls."  I can hear it now..."Hee hee...OMG this shirt is so freaking funny! I am totally going to buy it! giggle giggle giggle." 
"Mary Is My Homegirl" shirts were bad enough.