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Its true that semen attract ants. This is because semen contains fructose, a form of natural sugar.
Urine of a diabetic equally attract ants. So, the fact that your wives pant attracted ants doesn´t necessarily mean that she had sex with someone outside. I think its advisable to talk to her about your findings and ask her to see a Medical Doctor
for work-ups. This might help to discover a disease she has that she isn´t aware of.
Hope this will be of help to you