If by "illegal" you mean "against the law," then none. There is no state in the US where same-sex marriage is considered a crime. In fact, in most US states, same-sex marriage is legally recognized. (See related questions below: "What US states have legalized same-sex marriage" and "What US states have banned same-sex marriage.")
Yes, legal marriage in Canada is recognized in the United States. Before gay marriage was acceptable in the US, many people went to Canada to get married.
If the marriage was conducted legally in the other country, the US will recognize the marriage is valid.
Yes, a marriage in England is valid in the United States.
Same-sex marriage is legal in all 50 US states.
14 states plus Washington DC and 5 Native American tribes.
Yes. Your foreign marriage is legal in the United States.
Some US states and foreign countries have laws permitting same-sex marriage. Some US states and foreign countries have laws prohibiting same-sex marriage.
As of December 20, 2013, about 33% of US states have legalized same-sex marriage.
Same-sex marriage is legal in all US states effective June 26, 2015.
If by "illegal" you mean "against the law," then no. There is no state in the US where same-sex marriage is considered a crime. However, in some US states, same-sex marriage is not legally recognized. (See related questions below: "What US states have legalized same-sex marriage" and "What US states have banned same-sex marriage.")
ALL of them.