What a great point! Many Christians fail to consider that because multiple eggs need to be fertilized, artificial insemination is likely to result in the destruction or abandonment of the unborn child after conception. There is an inconsistency from a Christian perspective to taking a principled position on marriage but not on the unborn.
More here:
http://www.freerepublic.com/focus/f-news/2064555/posts
At the risk of sounding insensitive, some people are simply unable to have children and we MUST accept that this is as God intended it.
However, in the case of lesbians it is generally NOT an inability to have children, rather it is an UNWILLINGNESS to give up their sinful lifestyle.