My friend's husband is making passes at me

Jane and I have been friends from way back, since our first jobs after campus. She got married before me so I was involved in their wedding preparations, and so were they in our wedding a year later.

Her husband got close to mine because we practically became family friends; our children even go to the same school.

However, at the beginning of this year when I acquired a smart phone, my friend's husband started sending me weird WhatsApp messages, which turned erotic with time. He even started sexting me.

I'm so stressed; I get uncomfortable when we meet at our children's school. Actually, my husband noticed that I was avoiding him when we dropped our children at the beginning of term.

I have never done anything to seduce this guy, and as a matter of fact my friend looks more beautiful and attractive than me. 

I'm so afraid of telling my husband because he is capable of confronting him in the presence of my friend. What do I do? I'm so scared this may affect both our marriages.