You don't even have the opportunity to buy condoms, even if you choose to get ready for her that day, it is clear that you picked the worst option that day.If I remember right, to start the full NTR path, you have to refuse to buy the condoms. Then later, when she wants to have sex anyway, you need to pick something like "refuse to spend time with her" or maybe even "don’t have sex with her." I know that I had trouble with getting on that path because you have to make some really weird choices which go against you own interests.
The other option that put you on ntr is going to work... (and that choice isn't an excuse for her attitude)