How lord Shiva would know that this curse and/or boon is appropriate?
If someone prays to God, then God knows what that person wants:
As per chapter 4 of Kalki Purana:
Understanding that Kamala, whose face is exquisitely beautiful and who
is very dear to Lord Hari, had incarnated within this world, Sankara,
along with Parvarti, joyfully came to see her.
Lord Siva gives this boon:
О fortunate one, you will receive Lord Narayana as your husband. He
will joyfully accept your hand in marriage. Rest assured that there is
no other prince on earth who is a suitable match for you.
Other than this boon, Lord Shiva gives below curse to others
Anyone who lustily casts his glance upon you will immediately be
transformed into a woman, regardless of his age.
Whether they be demigods, demons, Nagas, Gandharvas, Caranas,
or anyone else, if they simply desire to enjoy a conjugal relationship with
you, they will immediately transform in to women
However, this curse is not applicable to Lord Hari, who is your only
husband. It is only applicable to others.
This must be an appropriate boon and/or curse as per Kalki Purana 4.45 (described in the later part of this answer).
Other incarnation of Goddess Lakshmi also worshiped Gauri and/or Shiva in order to get desired husband:
O mother Ambikā, wife of Lord Siva, I repeatedly offer my obeisances unto you, together with your children. May Lord Kṛṣṇa become my husband. Please grant this! - Śrīmad-Bhāgavatam (Bhāgavata Purāṇa) 10.53.46
Sita also prayed:
मज्जनु करि सर सखिन्ह समेता। गई मुदित मन गौरि निकेता।।
पूजा कीन्हि अधिक अनुरागा। निज अनुरूप सुभग बरु मागा।।
Having taken a dip into the lake with Her companions, Sita went with a glad heart to Girijå's temple. She offered worship with great devotion and begged of the Goddess a handsome match worthy of Her. - Ramcharitmanas 1.228
Why Padma stood too shy to ask for a boon when Shiva appears before her?
As per Kalki Purana 4.45
Having received her desired benediction from Lord Siva, Padmavati's
happiness knew no bound, so that her face blossomed brightly. She then
offered her obeisances to Uma and Mahadeva and returned home.
So it's clear that her prayers were to get desired husband. So a girl may feel shy in asking such boons. Also as God knows what a devotee is asking for so there is not need to ask explicitly.