Many companies, especially US based organisations, avoid allowing couples to work in the same team. There are many reasons for this;
1. Domestic Issues in the work place
As an example, I have personal experience of a team that included a couple. The couple in question brought their domestic arguments into the work place and pulled colleagues in to the disputes. This caused divisions in the team and distraction from the work.
In many cases relationships are short lived. Some may argue that it isn't worth disrupting a team and moving someone when a relationship may be short lived. However, unless the relationship ends amicably, the fallout from a split can be even worse and more disruptive to the working environment.
2. Potential for conflicts of interest
A couple are always likely to want to go on holiday together, so when it comes to fairly allocating holiday requests their Manager may be faced with a tricky problem on an ongoing basis. The interest of the couple conflicts with other members of the team and the Managers ability to manage resources smoothly. Alternatively, what if an opportunity for promotion comes up e.g. team leader, one of the couple is a prime candidate but appointing them would give them some form of managerial responsibility for their partner. Even the most professional of people might find it challenging not to favour their partner in the course of their duties.
3. Commercial Risks
Allowing couples to work in the same team could increase the risk and potential for fraudulent activity. Again I have real life experience of a couple who managed to falsify their sales figures. They colluded to gain unauthorised access to a system and change non-sales records to sales records. The activity was only discovered after a complaint from a customer. The couple were motivated by financial gain as the changes they made would improve their bonus payments.
Having a 'no relationship policy' may drive it underground but a carefully crafted policy is worth considering. You can design it so it intentionally drives short term relationships underground while encouraging longer term relationships to be declared. Those in short-term relationships are likely to behave more professionally to keep a lid on it and keep domestic stuff out of the work environment.
The key to implementing something like this is make sure your policy is clearly defined and properly communicated, ensuring your staff understand the reasons for implementation.
There are of course always the exception to the rule and some couples can work together without anyone being the wiser but as a Manager, I'd prefer not to have a couple in my Team. As a Romantic..........well that's a different story.
Hope this helps.