When thinking about these things, Japanese men seem to prefer going the safe route and opting for marriage with someone from their own nation.
If we look at the Ministry of Health, Labor and Welfare's figures for 20 (the most recent available), there were 236 cases where a British man married a Japanese woman in 2014, and 235 in 2015.
That says nothing directly about relationships per se, but it does weaken the barrier to spending one's life abroad with a special someone.
With this in mind, while it may take time for ideas to change within Japan, it seems probable that as more and more Japanese nationals spend time outside their home country, the number of international relationships and marriages may also increase. Is it only a matter of time before the sight of a Japanese guy with a Western girl becomes a common one on the streets of Tokyo?
After all, no relationship is perfect and perfection is boring!
The next time you're walking around in Tokyo, try this quick experiment: look for couples comprising a Western man and a Japanese woman, and count them. Now look for the opposite: Japanese men with Western women. If we look at official statistics for Japanese residents from Western countries (as of the 2016 census), men certainly outnumber women in Japan—by a ratio of almost 2-to-1 if we exclude short-term residents and diplomats (it's 3-to-2 if they're included).