Hi cakebump, you would find someone by Googling something like Paarberatung or Partnerberatung, or as outnumbered says, Lebensberatung. In most cases for couples therapy you would pay privately, and it could be up to 100 euros per session. There is provision in Germany for psychiatric therapy to be paid by your Krankenkasse, but I don't think that would apply for couples counselling. However, it might be worth phoning your Krankenkasse to see if they have a list of approved therapists - I did so recently for yoga teachers here in Berlin.
Things you need to watch out for: firstly many of the organisations offering counseling are religious - either the evangelische Kirche or the katholische Kirche, and their advice is likely to be swayed by the religious affiliation (which might not be a problem for you personally). Secondly, there's a real trend for alternative therapists (Heilpraktiker) to offer these sessions, and they're hardly qualified psychotherapists, are they?
This site has a number of addresses, and at least some of these will be able to provide you with a list of private counselors:
www.stadt-koeln.de/2/familie/rat-hilfe/01725/
This is an example of a private counselor:
www.paarberatungkoeln.de/
In a city as large as Cologne you are bound to find either a native English-speaking therapist or someone who speaks excellent English. This is one I found on the Net:
tanyawallmorris.de/services/paartherapie/
I have no experience of what things are like in Cologne, but you could try putting a question on Toytown in the local Cologne section, under Family Life, and see if anybody has personal experience of any of these providers. In any case, you need to check qualifications.
Best of luck with it all!