is being in relationship just a social construction?

so I've been single for 20 years now going to hit 21 years in March.

I don't really care about being single most of the time.

I enjoy the freedom of doing whatever I want.

But sometime I feel lonely. I feel like I want to be with someone.

So here comes the question,

Is being in relationship just a social construct? or is it ultimately what we, as human beings, always long for?

Throughout our life, we've all been taught (in many ways) that we all should get into a relationship and eventually get married. 

Well, getting married is definitely a social construct because marriage is itself a social term. Don't even get me started on marriage. haha.

We've watched movies where people (most of the time, a male protagonist and another female character) get in love and live happily ever after.

We've seen couples holding hands, hugging, kissing... you name it, on advertisements.

And most importantly, we've watched our parents, the closest couple you'd ever live with before you have your own relationship, living together.

All teach us either explicitly or implicitly that being in a relationship is one of the goal in our life or else you can't live normally. 

And of course when I say "living normally," it is referring to how most people in our society live their lives; it is the average lifestyle, the norm.

As I've pointed out, being in relationship sure does include a lot of social aspects.

I'm pretty sure there're a lot of emotional aspects to it as well (as a weak social constructionism).

Remember the times you feel like you want to be with someone, you feel like you want to tell someone about how you've been, you feel like you want to have a great time with someone? (apart from friends, I'm referring to someone "special")

It's really hard to distinguish between a social construct and a true human emotion when all of us have been living in the society for such a long time. We're all trained to think in a certain way.

What is the fundamental of being in a relationship anyways?

I personally believe there's only one fundamental and that is sex.

Yes, sex.

We're all animals and our goal is to reproduce.

I believe this is the most significant emotional aspect to being in a relationship.

Sex is clearly not a social construct as it is programmed in our brain since birth.

So next time when you're feeling lonely, it might be that you saw someone being in a relationship (or just the thought of living a "normal life" crosses your mind) or it might be that your "emotional drive" is urging you to do what you are supposed to do.

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also i just search online a little bit and found a forum talking specifically about monogamy and marriage
it's somehow related to what I've been talking about
here's the forum