As there are no ages given in this scenario (and hopefully Toby is above the legal age of consent!) some guesswork is required. If Walt and Jon's friendship extends to them meeting each other's families and given that they have known each other for over 20 years, Walt could have known Toby as a child. That fact alone could have freaked Jon out.
ilijah specifically asked if anyone was in the wrong. On the available info, I'd say Toby was. Why on earth would he want to tell his uncle that he'd been having sex with his uncle's friend
Gay marriage is still rather new in Europe - and stranger to many people outside of EU and North America.
There is 'a lot of' immigration of sexual minorities to Europe. Could imagine most of them don't come with a marriage in mind, but many dream of a union, to grow old with somebody, in a liberal atmosphere, not to mention far from war zones.
Christian is very cute and hot at the same time. I'd actually love to meet him and hang out with him with some fun. If only someone knew how to contact him.