In the epic of Gilgamesh Shamhat having sex with Enkidu is referred to as

she did for the man the work of a woman

But to love there some references too. In the end of tablet 1 Shamhat tells Enkidu of his destiny being "loved" by Gilgamesh "like a wife".

'Let [the people] see your face,

...... that exists I know indeed.

Go, Enkidu, to Uruk-the-Sheepfold,

where young men are girt with waistbands!

'Every day [in Uruk] there is a festival,

the drums there rap out the beat.

And there are harlots, most comely of figure,

graced with charm and full of delights.

'Even the aged they rouse from their beds!

o Enkidu, [as yet so] ignorant of life,

1 will show you Gilgamesh, a man happy and carefree,

look at him, regard his features!

'He is fair in manhood, dignified in bearing,

graced with charm is his whole person.

He has a strength more mighty than yours,

unsleeping he is by day and by night.

'0 Enkidu, cast aside your sinful thoughts!

Gilgamesh it is whom divine Shamash loves.

The gods Anu, Enlil and Ea have broadened his wisdom.

Before you even came from the uplands,

Gilgamesh in Uruk was seeing you in dreams:

Gilgamesh rose to relate a dream, saying to his mother:

"0 mother, this is the dream 1 had in the night-, "The stars of the heavens appeared above me,

like a rock from the sky one fell down before me.

1 lifted it up, but it weighed too much for me,

1 tried to roll it, but 1 could not dislodge it.

"'The land of Uruk was standing around it,

[the land was gathered] about it.

A crowd [was milling about] before it,

[the menfolk were] thronging around it.

'''[Like a babe-in]-arms they were kissing its feet,

like a wife [I loved it,] caressed and embraced it.

[I lifted it up,] set it down at your feet,

[and you, 0 mother, you] made it my equal."

'[The mother of Gilgamesh] was clever and wise,

well versed in everything, she said to her son -[Wild-Cow] Ninsun was clever and wise,

well versed in everything, she said to Gilgamesh:

'''The stars of heaven [appeared] above you,

[like a] rock from the sky one fell down before you.

You lifted it up, but it weighed too much for you,

you tried to roll it, but you could not dislodge it.

, "You lifted it up, set it down at my feet,

and I, Ninsun, 1 made it your equal.

Like a wife you loved it, caressed and embraced it:

a mighty comrade will come to you, and be his friend's saviour.

, "Mightiest in the land, strength he possesses,

his strength is as mighty as a rock from the sky.

Like a wife you'll love him, caress and embrace him,

he will be mighty, and often will save you."

'Having had a second dream,

he rose and entered before the goddess, his mother.

Said Gilgamesh to her, to his mother,

~'Once more, 0 mother, have I had a dream -"'[In a street] of Uruk-the-Town-Square,

an axe was lying with a crowd gathered round.

The land [of Uruk] was standing around it,

[the country was] gathered about it.

, "A crowd was milling about before it,

[the menfolk were] thronging around it.

I lifted it up and set it down at your feet,

like a wife [I loved] it, caressed and embraced it,

[and you, 0 mother,] you made it my equal."

'The mother of Gilgamesh was clever and wise,

well versed in everything, she said to her son -Wild-Cow Ninsun was clever and wise,

well versed in everything, she said to Gilgamesh:

, "My son, the axe you saw is a friend,

like a wife you'll love him, caress and embrace him,

and I, Ninsun, I shall make him your equal.

A mighty comrade will come to you, and be his friend's saviour,

mightiest in the land, strength he possesses,

his strength is as mighty as a rock from the sky."

'Said Gilgamesh to her, to his mother,

"May it befall me, 0 mother, by Counsellor Enlil's

command!

Let me acquire a friend to counsel me,

a friend to counsel me I will acquire!"

[So did Gilgamesh] see his dreams.'

[After] Shamhat had told Enkidu the dreams of Gilgamesh,

the two of them together [began making] love

In further plot we can deduce Gilgamesh and Enkidu are not just having a fling. They are homies for life which drives the goddess Ishtar crazy with jealousy because she fell in love with Gilgamesh as well.

I don't know if it is the oldest reference to love. It is the 18th century BC. Egyptians left some literature that is even older than that. You can bet it features love as well.

The Gilgamesh translation is by Andrew George. The penguin classics edition is split into tablets so it won't be too hard to find the Sumerian and the Akkadian expressions used to describe their loves.