While there is certainly a bug involved, you should be fine if you define a proper /listsize in your <dropdowns>:
<dropdown sex>
/ caption = "Sex"
/ options = ("female", "male")
/ listsize = (15%,10%)
</dropdown>
<textbox age>
/ caption = "Age"
/ mask = positiveinteger
/ range = (7, 110)
</textbox>
<surveypage demographics>
/ caption = "Please answer the following demographic questions"
/ fontstyle = ("Verdana", -14, true, false, false, false, 5, 0)
/ questions = [1=sex; 2=age]
</surveypage>
<survey demographics>
/ pages = [1=demographics]
/ responsefontstyle = ("Verdana", -12, false, false, false, false, 5, 0)
/ itemfontstyle = ("Verdana", -13, false, false, false, false, 5, 0)
/ itemspacing = 2%
/ showpagenumbers = false
</survey>