This is one of the biggest confusions that there was no Goddess Lakshmi before Samudra Manthana.
Goddess Lakshmi was actually daughter of Bhrigu & Khyati earlier and got married to Lord Vishnu. Indra showed disrespect to her (or sage Durvasa in some stories) and she made her abode in the abysmal depth of the sea and later emerged during Samudra Manthan. Read in more detail about other incarnations of Goddess Lakshmi from Vishnu Purana.
As per SATI KHAND and PARVAT KHAND of Shiva Purana:
Goddess Sati gave up her body in Daksha Yajna and born to other parents (Himalaya and Maina.) as Parvati. It was the grief of Sati's death that led Shiva to exact vengeance. Later, Shiva had no interest in worldly life.
But Tarakasur got a boon from Lord Bhrahma that
except Shiva's son nobody should be able to kill him.
When Tarakasur became biggest threat to Indra, Indra called Kamadeva and told him that
The demon king Tarakasur could be killed only by such a person who was the son of Shiva. Indra instructed Kamadeva to arouse passion in lord Shiva, so that he agrees to marry Parvati.
But when Kamdeva was trying his best, the third eye of lord Shiva got opened and Kamadeva was reduced to ashes. After that Narada preaches and Parvati does penance.
So it was different story altogether for Goddess Parvati.
But Goddess Lakshmi didn't give up her body, she just changed her abode. So, she was already married to Lord Vishnu. But in Samudra Manthana, not only Vishnu (in his Kurma form) but also Devas and Asuras were engaged and all had equal right on whatever emerged from that exercise. So in my opinion, Goddess Lakshmi had to do that Lila (of choosing a husband) but after this point this question is exactly duplicate of Why did Goddess Lakshmi choose Lord Vishnu as her consort? so you may find better answers there. And one more point, during Samudra Manthana no body denied if something was given after mutual consent, even Lord Shiva accepted Halahala.
