Which scripture mention about marriage between brahma and saraswati?
Lord Brahma married to Devi Saraswati is described in below verse of Chapter Three of Matsya Purana.
एवमुक्तास्ततः सर्वे ससृजुर्विविधाः प्रजाः।
गतेषु तेषु सृष्ट्यर्थं
उपयेमे स विश्वात्मा शतरूपामनिन्दिताम् ।।
Matsya Purana 3.42 ।।
At this command of the father , the sons resorted to several types of
creations. After the departure of the mind-born sons of Brahma ,after
offering their salutation to the father , then the lord married
Satarupa who had been lying over his feet.
Apparently Brahma was condemned by his sons for lusting after saraswati, but how after that did Brahma end up marrying saraswati?
It's said in the Matsya purana that Devi Saraswati was born from limbs of Lord Brahma , and their union is also conceived both visible and invisible. It's also said that Saraswati is bound to be there ,wherever Brahma will be and Brahma will be their where Saraswati will be . They are inseparable from beginning .
And Gayatri can not leave the compny of Brahma.
So according to scripture itself the union of Brahma and Saraswati has two meaning one is Vyavaharika (visible ) and Other is divine (invisible). i.e. symbolic.
The answer is given in Matsya Purana -chapter 4 - Story of Saraswati.
अन्यच्च सर्ववेदानामधिष्ठाता चतुर्मुखः।
ब्रह्मणस्तद्वदङ्गभूता निगद्यते।। 4.7।।
विरिञ्चिर्यत्र भगवांस्तत्र देवी सरस्वती।।
भारती यत्र यत्रैव तत्र तत्र प्रजापतिः।। 4.8 ।।
The other reason is that as Brahma happens to be creator of vedas
.Similarly Gayatri in the form of Satarupa is said to have born of the
limbs of Brahma , therefore this union is conceived as visible and
invisible .Alas whenever there is Brahma ,Saraswati is bound to be
there. And whenever there is Saraswati , Brahma must be there.
यथा तपो न रहितश्छायया दृश्यते क्वचित्।
गायत्री ब्रह्मणः पार्श्वं
तथैव न विमुञ्चति।। 4.9 ।।
As the sunshine cannot be separated from the sun ,similarly Gayatri
cannot leave the company of Brahma.
Matsya Purana also gives us advice that the activities of Gods proper or Improper do not generate any good or bad fruits. i.e. their Karmas do not affect them as our karmas.
कार्य्याकार्ये न देवानां शुभाशुभफलप्रदे। यस्मात्तस्मान्न राजेन्द्र!
तद्विचारो नृणां शुभः।। 4.6 ।।
O great king ,since the activities of (irrespective of their being )
proper or improper , do not yield good or bad fruits ,therefore it
would not be proper for the humans to think about them.
Conclusion - So considering these above points that Brahma and Saraswati are bound to be at same place i.e. they are inseparable ,
and can not leave each other . And are always in union , Brahmas
marriage with Saraswati proved to be valid and that is the reason why
Lord Brahma married Devi Saraswati.
Symbolic Meaning of their marriage -
The divine couple of Brahma and Saraswati represent the mind, knowledge and intellect. They are both often depicted with the Lotus flower which symbolises knowledge and supreme reality.
NOTE- -Gayatri , Satarupa , are another names of Devi Saraswati herself.