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Try this. Use p1=p1+ ... instead of p1 = ...

t=var('t')
for i in range(-3,3,1):
    p1 = p1+parametric_plot((t,i),(t,-5,5),color="red")
    p2 = p2+parametric_plot((i,t),(t,-5,5),color="gray")
show(p1+p2)