Michael Hannon offers a clever analogy to make a profound point about definitions and reality:
"...By way of illustration, let me return to the natatorium. Water polo is a sport in which seven players swim around in a deep pool, cooperating in an effort to beat the opposing team while abiding by the rules of the game. A water polo team, then, is simply a group of people who play this sport with one another, who band together to win water polo games against other teams. Thus playing water polo is the characteristic activity of a water polo team, meaning not only that it reveals or witnesses to what this team is, but also that it actually makes this team what it is. It is in virtue of their playing water polo that a group of people is a water polo team at all.
Given that, it is obviously insufficient for forming a water polo team that a group of seven people just swim laps around the pool during what would otherwise be a water polo game. To qualify as this particular type of association, the seven must actually do the thing that makes them this type of association. To be a water polo team, the group must play water polo. This also means that, were the seven for some reason incapable of doing the activity characteristic of such team-ship, they could not be a water polo team. Seven people who cannot swim cannot form a team.
What does any of this have to do with marriage? As with water polo, to get at what marriage is, we should ask what the characteristic activity of marriage is. As nearly every society in history has recognized, marriage’s characteristic action is sexual intercourse. Thus coitus is classically termed the “marital act”, and it has traditionally been said to “consummate” the marriage, in a way that no other act—sexual or otherwise—can." (First Things)