[R] GLM contrasting question

[sodiumfish]

I have run a glm with a final formula of : (dependent variable = parasite
load, main effects are sex, month, length and weight, with sex:month and
length:weight first order interactions).

I am using the summary(mod) command to give me the contrasts, which I
believe use the contr.treatment command. I do not have a treatment group as
such as I am comparing data from a wild system so I use the relevel command
to reorder my factors in order to check the difference between each level
and every other. I then use the coefficients and their related p-values to
assess whether each level of my factors is significantly different from the
next. This is fine for most things but  what I really want to do is to
assess whether there is a significant difference of between males and
females in any particular month. However, because of my interaction term the
male and female for any particular month are always the missing coefficients
and so I can't contrast them with one another.

Is there a way (preferably a relatively simple way) for me to do this. You
will probably realise from my description above that I am a biologist not a
statistician, so if anyone can help me in plain English that would be an
enormous help.

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