That doesn't make you bad though. That is just your preference. If I may share my own personal experience, long lasting relationships are not based on sexual attraction, though its obviously important.
Don't get me wrong, sexual attraction is still important years and decades in, but in the end, love is a choice and whether they have big tits or not, you will always have to choose love (when the honeymoon period wears off).
Trust me, you can be with a woman who has big tits that you don't get along well with over time... her having big tits won't matter much then.
If you can get along and they have big tits over the long passage of time -- that's a wonderful thing
In the end, we're all people who have needs in far more areas than sex. I wish you genuine good luck!