Crowhugger, I'm taking you at your word, (although I wonder if you're just teasing by saying you don't understand why it offends)
Imagine if you will driving along and passing a large billboard on which was depicted an enormous photo of your mother. The photo has been digitally altered so she appears to be wearing, say, a sexy thong and boob-tassels and she's holding an opened bottle of whiskey and winking. People are laughing at her, as some of them recognise the lady as your mum. Later, some even come up to you leering and ask if you've seen it. I do think you'd be deeply upset and offended, and want the poster taken down. You'd probably be very angry as well. The Virgin Mary is not, of course, my mother, but even more importantly, she is/was the Mother of Our Lord, the Mother of God. She should not be depicted holding a pregnancy test and looking peeved. She is not an object of derision or disrespect. Maybe this can help you to understand?