Is it ethical to apply for a short-term grant with a partner/spouse/girlfriend? My partner and I live in different countries, and we have been discussing a research topic for some time but have had difficulty making progress due to the distance, local responsibilities, etc.  We have recently found a visiting researcher grant that would allow me to visit her for a couple months and work exclusively on our project.  We would both continue to receive salaries from our respective institutions (the grant requires this), and we plan to request only enough funding for travel and basic living expenses (i.e., a round-trip flight, daily meals, and daily transport; the latter two, because her country is more expensive than mine).

Are there any ethical issues with us applying for such a grant?  We do not, for example, plan to request housing funds, since I will stay with her.  We are not sure whether we should disclose our relationship status in the application, because it's a small grant, and it could be seen as distracting or superfluous information. (See also the discussion under @DanRomik's answer.)

Edit: to be clear, we are each qualified in our fields to carry out such a project.  Indeed, we've already been carrying out the project, but at a slow pace, and the main goal is to have a couple months of focused effort on the project.
Provided you are qualified and intend to do a good job under the grant, I see no ethical considerations at all. Many people (most?) applying for grants in other countries have a variety of goals, not all of which are relevant to the position itself. Wanting to live in Paris is pretty common, I'd guess. 

There would be an issue if she is the decision maker on who gets the grant. In such a case she should recuse from the decision and pass it to others, such as a superior. But otherwise, there should be no problem.