Now, I will outline three reasons why you shouldn't do it.
1. You're not putting God first in your worship.
You know why you don't take a first date to a movie? Because you can't talk through a movie; you don't get to know the person during the movie, except for whether they're squeamish at the sight of blood or something. No, the point of going to the movies is to see the movie. Christians go to church to worship God. Yes, there is fellowship (gathering with friends) that happens but this is not the priority.
This is actually part of the challenge with youth ministry. We build it around friends and events, without centering around the gospel and what Christ has done. It gets to the point that if a youth's friend isn't there that week, they skip out also. Not cool bro.
2. You're not trusting in God's sovereignty.
As one tweeter put it, "you call him your Creator but you can't trust him with your life?"
A true Christian surrenders all to the One who made them. Now, in terms of dating the internet is filled with articles of "how to find the one for you" - but really, what you're saying is either "God is taking too long, I'm going to hurry up and find me someone now" or "God forgot about me, I'm going to solve this on my own."
Have you ever heard, "the right thing at the wrong time is still the wrong thing." Trusting in God proves to be such a thrill, that when the right person comes along it truly feels right.
3. You're not doing yourself or her/him any favours. Which really recaps my first two points.
You can turn the tables and give reasons why it is okay, but in doing so I hope you are examining your heart motive.
Important: What I'm saying is don't make visiting another church service in order to "meet someone" be your first, only, or main reason for visiting. If do meet someone and something beautiful and lasting comes from that, perfect. Just be careful that you are not Ideal Spouse Shopping (probably a better title for this post).