Anal and vaginal warts are only transmitted sexually. His reluctance to believe you is a worriesome sign. If you suspect an affair, you are probably correct. Genital warts, wherever they are, are caused by HPV. HPV is a systemic disease, so, warts can show up anywhere, but most commonly are seen on the vulva, perineum, anus, cervix.
There is not really any way to determine when someone was infected, and yes, it could have been 18 years ago. If you were my patient, I would refer you to the best proctologist that I could find. I'm not totally sure what the recommendation for colonoscopy/anoscopy, etc is. I'm sure the wart could be biopsied. Also, HPV typing is available to determine whether you have types of the virus that are high risk for causing cancer.
Finally, there is a nice medication called Aldara that can be applied topically to the wart to shrink it.
I hope this helps!
Just wanted to let you know that I have had only ONE Faithful sex partner in my entire life of over 50 years and I have high risk hpv which cannot be seen. I was not his first. If you have no reason to suspect him of cheating, maybe he isn't.