It is because long and short you would be showing support to an invalid priesthood.
What partaking would or would not show depends a bit on circumstances. But of course a lie from politeness would still be a lie.
Church Militant:
They may be sacraments, in fact Lutherans profess them to be so, and even their rites may be accurate, but they still are not valid…for reasons already given.
That is the core of the problem, how can what the lutherans do/offer be sacraments if they are not valid?
One cannot offer gold if one only has fool’s gold. A rule requiring then “Do not take gold from him.” is then not meaningful as it is worded. It could have at least two meanings either “Do not take fool’s gold from him.” or “Do not try to get real gold from him.”. Which it is depends on context.
Therefore the sentence
“Catholics may not receive communion in those communities which lack a valid sacrament of Orders.”
is not meant literally, because:
oce.catholic.com/index.php?title=Holy_Communion
“By Communion is meant the actual reception of the Sacrament of the Eucharist.”
And for something to be a sacrament:
oce.catholic.com/index.php?title=Sacraments
“In every sacrament three things are necessary: the outward sign; the inward grace; Divine institution.”
The lutherans lack Divine institution, it is lacking in their bread sharing. Therefore it lacks something necessary for it being a sacrament. Therefore it is not communion because it cannot be the reception of a sacrament.
Therefore the statement:
Catholics (and anybody else) cannot receive communion in a lutheran service.
is a true statement, at least if assuming the statements of that catholic encyclopedia are correct.
And therefore the rule:
“Catholics may not receive communion in those communities which lack a valid sacrament of Orders.”
is on its own not clear, because forbidding what is impossible is pointless, it normaly means that the attempt is forbidden.
And regarding obeying the Church, that i am required to do. But i am not required to obey your interpretation of what the Church says, if i am uncertain whether your interpretation is correct.
@Luvtosew
Acting only upon what is feeling right is wrong, if something is forbidden. And what Church Militant cited can indeed mean its forbidden, its just not that clear from my point of view, because from the wording it seems to forbid something, which is impossible. But that only means that something is forbidden, just not the literal thing, which does not require to be forbidden, because its impossible.
At least undoubtly attending such a lutheran service and partaking fully could be a lie.