There are quite a few people who come and go around here that, like you, only want to date as a couple. The problem with this goal is that it is often difficult to find people who are interested in two people and are okay with the preconceived 2+1 (or 2+2) attitude that often comes with it. Instead of feeling like it is 1+1+1 where everyone is equal, there is often an idea that the original couple is more important and that the relationship between the two is sacred whereas the relationship(s) with the third person is destined to always be of lesser consequence.
Group relationships are also more difficult many times because of the sheer number of relationships that are within them. If you two start dating someone, you have you+original partner, you+new partner, new partner+original partner, and all three of you together to maintain. Add a fourth person if you meet another couple and you add a bunch more relationships.
You also have to face the idea of the person(s) initially falling for both of you, but eventually coming to love only one while being fond of the other as a friend or possibly vaguely romantically without "loving" whole-heartedly. You can't force a person to care more than they care, so would the relationship end as soon as it isn't equal for both of you or would it be allowed to go on as more of a vee (two people involved with one person without being involved with each other) instead of a triad (all three involved with one another)?
So... It's complicated. You CAN date as a couple, but be prepared for a lot of struggles. Generally people find it easier to date separately, and if a triad or quad happens to develop, view it as an awesome bonus.